Fast page loading by moving ASP.NET AJAX scripts after visible content

ASP.NET ScriptManager control has a property
LoadScriptsBeforeUI, when set to false, should
load all AJAX framework scripts after the content of the page. But
it does not effectively push down all scripts
after the content. Some framework scripts, extender scripts and
other scripts registered by Ajax Control Toolkit still load before
the page content loads. The following screen taken from shows several
script tags are still added at the beginning of

forces them to download first before the page content is loaded and
displayed on the page. Script tags pause rendering on several
browsers especially in IE until the scripts download and execute.
As a result, it gives user a slow loading impression as user stares
at a white screen for some time until the scripts before the
content download and execute completely. If browser could render
the html before it downloads any script, user would see the page
content immediately after visiting the site and not see a white
screen. This will give user an impression that the website is
blazingly fast (just like Google homepage) because user will
ideally see the page content, if it’s not too large, immediately
after hitting the URL.

Figure: Script blocks being delivered before the content

From the above screen shot you see there are some scripts from
ASP.NET AJAX framework and some scripts from Ajax Control Toolkit
that are added before the content of the page. Until these scripts
download, browser don’t see anything on the UI and thus you get a
pause in rendering giving user a slow load feeling. Each script to
external URL adds about 200ms avg network roundtrip delay outside
USA while it tries to fetch the script. So, user basically stares
at a white screen for at least 1.5 sec no matter how fast internet
connection he/she has.

These scripts are rendered at the beginning of form tag
because they are registered using
Page.ClientScript.RegisterClientScriptBlock. Inside
Page class of System.Web, there’s a method
BeginFormRender which renders the client script blocks
immediately after the form tag.

   1: internal void BeginFormRender(HtmlTextWriter writer, string formUniqueID)
   2: {
   3:     ...
   4:         this.ClientScript.RenderHiddenFields(writer);
   5:         this.RenderViewStateFields(writer);
   6:         ...
   7:         if (this.ClientSupportsJavaScript)
   8:         {
   9:             ...
  10:             if (this._fRequirePostBackScript)
  11:             {
  12:                 this.RenderPostBackScript(writer, formUniqueID);
  13:             }
  14:             if (this._fRequireWebFormsScript)
  15:             {
  16:                 this.RenderWebFormsScript(writer);
  17:             }
  18:         }
  19:         this.ClientScript.RenderClientScriptBlocks(writer);
  20: }

Figure: Decompiled code from System.Web.Page class

Here you see several script blocks including scripts registered
by calling ClientScript.RegisterClientScriptBlock are
rendered right after form tag starts.

There’s no easy work around to override the
BeginFormRender method and defer rendering of these scripts.
These rendering functions are buried inside System.Web and
none of these are overridable. So, the only solution seems to be
using a Response Filter to capture the html being written and
suppress rendering the script blocks until it’s the end of the
body tag. When the tag is about to be
rendered, we can safely assume page content has been successfully
delivered and now all suppressed script blocks can be rendered at

In ASP.NET 2.0, you to create Response Filter which is an
implementation of a Stream. You can replace default
Response.Filter with your own stream and then ASP.NET will
use your filter to write the final rendered HTML. When
Response.Write is called or Page’s Render method
fires, the response is written to the output stream via the filter.
So, you can intercept every byte that’s going to be sent to the
client (browser) and modify it the way you like. Response Filters
can be used in variety ways to optimize Page output like stripping
off all white spaces or doing some formatting on the generated
content, or manipulating the characters being sent to the browser
and so on.

I have created a Response filter which captures all characters
being sent to the browser. It it finds that script blocks are being
rendered, instead of rendering it to the
Response.OutputStream, it will extract the script blocks out
of the buffer being written and render the rest of the content. It
stores all script blocks, both internal and external, in a string
buffer. When it detects tag is about to be
written to the response, it flushes all the captured script blocks
from the string buffer.

   1: public class ScriptDeferFilter : Stream
   2: {
   3:     Stream responseStream;
   4:     long position;
   6:     /// 
   7:     /// When this is true, script blocks are suppressed and captured for 
   8:     /// later rendering
   9:     /// 
  10:     bool captureScripts;
  12:     /// 
  13:     /// Holds all script blocks that are injected by the controls
  14:     /// The script blocks will be moved after the form tag renders
  15:     /// 
  16:     StringBuilder scriptBlocks;
  18:     Encoding encoding;
  20:     public ScriptDeferFilter(Stream inputStream, HttpResponse response)
  21:     {
  22:         this.encoding = response.Output.Encoding;
  23:         this.responseStream = response.Filter;
  25:         this.scriptBlocks = new StringBuilder(5000);
  26:         // When this is on, script blocks are captured and not written to output
  27:         this.captureScripts = true;
  28:     }

Here’s the beginning of the Filter class. When it initializes,
it takes the original Response Filter. Then it overrides the
Write method of the Stream so that it can capture the
buffers being written and do it’s own processing.

   1: public override void Write(byte[] buffer, int offset, int count)
   2: {
   3:     // If we are not capturing script blocks anymore, just redirect to response stream
   4:     if (!this.captureScripts)
   5:     {
   6:         this.responseStream.Write(buffer, offset, count);
   7:         return;
   8:     }
  10:     /* 
  11:      * Script and HTML can be in one of the following combinations in the specified buffer:          
  12:      * .....< script ....>..........
  13:      * < script ....>..........
  14:      * < script ....>.....
  15:      * < script ....>..... .....
  16:      * ....< script ....>..... 
  17:      * < script ....>..... 
  18:      * ..........
  19:      * .....
  20:      * < script>.....
  21:      * .... 
  22:      * ......
  23:      * Here, "...." means html content between and outside script tags
  24:     */
  26:     char[] content = this.encoding.GetChars(buffer, offset, count);
  28:     int scriptTagStart = 0;
  29:     int lastScriptTagEnd = 0;
  30:     bool scriptTagStarted = false;
  32:     for (int pos = 0; pos < content.Length; pos++)
  33:     {
  34:         // See if tag start
  35:         char c = content[pos];
  36:         if (c == '<')
  37:         {
  38:             int tagStart = pos;
  39:             // Check if it's a tag ending
  40:             if (content[pos+1] == '/')
  41:             {
  42:                 pos+=2; // go past the 
  44:                 // See if script tag is ending
  45:                 if (isScriptTag(content, pos))
  46:                 {
  47:                     /// Script tag just ended. Get the whole script
  48:                     /// and store in buffer
  49:                     pos = pos + "script>".Length;
  50:                     scriptBlocks.Append(content, scriptTagStart, pos - scriptTagStart);
  51:                     scriptBlocks.Append(Environment.NewLine);
  52:                     lastScriptTagEnd = pos;
  54:                     scriptTagStarted = false;
  55:                     continue;
  56:                 }
  57:                 else if (isBodyTag(content, pos))
  58:                 {
  59:                     /// body tag has just end. Time for rendering all the script
  60:                     /// blocks we have suppressed so far and stop capturing script blocks
  62:                     if (this.scriptBlocks.Length > 0)
  63:                     {
  64:                         // Render all pending html output till now
  65:                         this.WriteOutput(content, lastScriptTagEnd, tagStart - lastScriptTagEnd);
  67:                         // Render the script blocks
  68:                         byte[] scriptBytes = this.encoding.GetBytes(this.scriptBlocks.ToString());
  69:                         this.responseStream.Write(scriptBytes, 0, scriptBytes.Length);
  71:                         // Stop capturing for script blocks
  72:                         this.captureScripts = false;
  74:                         // Write from the body tag start to the end of the inut buffer and return
  75:                         // from the function. We are done.
  76:                         this.WriteOutput(content, tagStart, content.Length - tagStart);
  77:                         return;
  78:                     }
  79:                 }
  80:                 else
  81:                 {
  82:                     // some other tag's closing. safely skip one character as smallest
  83:                     // html tag is one character e.g. . just an optimization to save one loop
  84:                     pos++;
  85:                 }
  86:             }
  87:             else
  88:             {
  89:                 if (isScriptTag(content, pos+1))
  90:                 {
  91:                     /// Script tag started. Record the position as we will 
  92:                     /// capture the whole script tag including its content
  93:                     /// and store in an internal buffer.
  94:                     scriptTagStart = pos;
  96:                     // Write html content since last script tag closing upto this script tag 
  97:                     this.WriteOutput(content, lastScriptTagEnd, scriptTagStart - lastScriptTagEnd);
  99:                     // Skip the tag start to save some loops
 100:                     pos += "< script".Length;
 102:                     scriptTagStarted = true;
 103:                 }
 104:                 else
 105:                 {
 106:                     // some other tag started
 107:                     // safely skip 2 character because the smallest tag is one character e.g. 
 108:                     // just an optimization to eliminate one loop 
 109:                     pos++;
 110:                 }
 111:             }
 112:         }
 113:     }
 115:     // If a script tag is partially sent to buffer, then the remaining content
 116:     // is part of the last script block
 117:     if (scriptTagStarted)
 118:     {
 120:         this.scriptBlocks.Append(content, scriptTagStart, content.Length - scriptTagStart);
 121:     }
 122:     else
 123:     {
 124:         /// Render the characters since the last script tag ending
 125:         this.WriteOutput(content, lastScriptTagEnd, content.Length - lastScriptTagEnd);
 126:     }
 127: }

There are several situations to consider here. The Write
method is called several times during the Page render process
because the generated HTML can be quite big. So, it will contain
partial HTML. So, it's possible the first Write call contains a
start of a script block, but no ending script tag. The following
Write call may or may not have the ending script block. So, we need
to preserve state to make sure we don't overlook any script block.
Each Write call can have several script block in the buffer
as well. It can also have no script block and only page

The idea here is to go through each character and see if there's
any starting script tag. If there is, remember the start position
of the script tag. If script end tag is found within the buffer,
then extract out the whole script block from the buffer and render
the remaining html. If there's no ending tag found but a script tag
did start within the buffer, then suppress output and capture the
remaining content within the script buffer so that next call to
Write method can grab the remaining script and extract it
out from the output.

There are two other private functions that are basically helper
functions and does not do anything interesting:

   1: private void WriteOutput(char[] content, int pos, int length)
   2: {
   3:     if (length == 0) return;
   5:     byte[] buffer = this.encoding.GetBytes(content, pos, length);
   6:     this.responseStream.Write(buffer, 0, buffer.Length);
   7: }
   9: private bool isScriptTag(char[] content, int pos)
  10: {
  11:     if (pos + 5 < content.Length)
  12:         return ((content[pos] == 's' || content[pos] == 'S')
  13:             && (content[pos + 1] == 'c' || content[pos + 1] == 'C')
  14:             && (content[pos + 2] == 'r' || content[pos + 2] == 'R')
  15:             && (content[pos + 3] == 'i' || content[pos + 3] == 'I')
  16:             && (content[pos + 4] == 'p' || content[pos + 4] == 'P')
  17:             && (content[pos + 5] == 't' || content[pos + 5] == 'T'));
  18:     else
  19:         return false;
  21: }
  23: private bool isBodyTag(char[] content, int pos)
  24: {
  25:     if (pos + 3 < content.Length)
  26:         return ((content[pos] == 'b' || content[pos] == 'B')
  27:             && (content[pos + 1] == 'o' || content[pos + 1] == 'O')
  28:             && (content[pos + 2] == 'd' || content[pos + 2] == 'D')
  29:             && (content[pos + 3] == 'y' || content[pos + 3] == 'Y'));
  30:     else
  31:         return false;
  32: }

The isScriptTag and isBodyTag functions may look
weird. The reason for such weird code is pure performance. Instead
of doing fancy checks like taking a part of the array out and doing
string comparison, this is the fastest way of doing the check. Best
thing about .NET IL is that it's optimized, if any of the condition
in the && pairs fail, it won't even execute the rest. So,
this is as best as it can get to check for certain

There are some corner cases that are not handled here. For
example, what if the buffer contains a partial script tag
declaration. For example, ".... and that's it. The
remaining characters did not finish in the buffer instead next
buffer is sent with the remaining characters like "ipt src="..."
>...... In such case, the script tag won't be
taken out. One way to handle this would be to make sure you always
have enough characters left in the buffer to do a complete tag name
check. If not found, store the half finished buffer somewhere and
on next call to Write, combine it with the new buffer sent and do
the processing.

In order to install the Filter, you need to hook it in in the
Global.asax BeginRequest or some other event that's fired before
the Response is generated.

   1: protected void Application_BeginRequest(object sender, EventArgs e)
   2: {
   3:     if (Request.HttpMethod == "GET")
   4:     {
   5:         if (Request.AppRelativeCurrentExecutionFilePath.EndsWith(".aspx"))
   6:         {
   7:             Response.Filter = new ScriptDeferFilter(Response);
   8:         }
   9:     }
  10: }

Here I am hooking the Filter only for GET calls to
.aspx pages. You can hook it to POST calls as well. But
asynchronous postbacks are regular POST and I do not want to
do any change in the generated JSON or html fragment. Another way
is to hook the filter only when ContentType is

When this filter is installed, defers all
script loading after the

tag completes.

Figure: Script tags are moved after the

tag when the
filter is used

You can grab the Filter class from the
App_CodeScriptDeferFilter.cs of the Dropthings project. Go
to CodePlex site
and download the latest code for the latest filter.

21 thoughts on “Fast page loading by moving ASP.NET AJAX scripts after visible content”

  1. How can you measure the difference between this two solutions? Is there any way to measure the time which is passed between the request for the page and the moment when the page starts to render?

  2. I haven’t found any tool that can hook on browser and see when it starts rendering. So, I rely on my eye for now. As my goal is to improve perceived speed, not the actual speed, I have to look at the site and see how fast it ‘feels’.

  3. Good Idea Omar, but I think that you are serving the client at the expense of the server.

    You added so much processing for filtering each page request that may lead to performance issue on server during heavy load.

    Did you captured the time consumed by your new filter approach and compared it with the 1.5 sec?

  4. if you use HttpWatch Professional, it shows all active connections used by the browser client. It wont show you exactly when the form tag will begin rendering, but you will be able to see the page download first and begin loading page elements such as images or javascripts. In the realtime transcript, it becomes quite clear that loading all js at the end makes for very fast page loading.

    At, we had to do something about multiple css files called at the start of the page. The same techniques / filter can be used to consolidate all css into one single file, compress and then serve. We do it via an offline process, but it can be done automatically with this technique.

    Lots of high profile system (such as can get a nice boost with this.

    Thanks for sharing all your web dev. IQ

  5. On my laptop, it takes about an avg 40ms more to filter out 20KB of html output. It’s negiligble compared to the client side benefit we get. There can be 20% reduction on total RPS on the server, but we can always throw in more server power or optimize server side code to compensate for that small loss in RPS.

  6. Omar,

    I was trying to implement this conecept on my application i have a auto complete text search box and the user types in by the time the page renders how to handle this scenarious and sometimes the output also behaves weirdly. Please update

  7. I was trying to implement this conecept on my application thank u

  8. Adsense for ScriptDeferFilter v2:

    // See if script tag is ending

    if (isScriptTag(content, pos))


    /// Script tag just ended. Get the whole script

    /// and store in buffer

    pos = pos + “script>”.Length;

    string temp=new string(content, scriptTagStart, pos – scriptTagStart);

    if ((temp.IndexOf(“google_ad_client”)==-1) && (temp.IndexOf(“googlesyndication”) == -1))


    scriptBlocks.Append(content, scriptTagStart, pos – scriptTagStart);





    this.WriteOutput(content, scriptTagStart, pos – scriptTagStart);


    lastScriptTagEnd = pos;

    scriptTagStarted = false;

    pos–; // continue will increase pos by one again



  9. Omar,

    It is really very good article. Just need to know how can I implement ScriptDeferFilter.cs on VS 2005 project .

  10. There is a problem with scriptdeferfilter.cs my ajax rating is not working with it.

    can u post the fix.

  11. Hi Omar,

    Thanks for the article. I'm using it in one of my projects, but I have found something strange

    “I have a couple of pages that only have the google analytics JS, at the end of the page, when I applied the ScriptDeferFilter the google analytics JS disappear”

    Any insights?

  12. Hi Omar “again”

    If I put the Google Analytics JS before the “head tag” ends it works grate, but if I put it after the “body tag” ends the ScriptDeferFilter eliminates it.

    In summary,

    if you use ScriptDeferFilter you must place at least one script before the “head tag” ends. If you manually place them after the “body tag” ends the ScriptDeferFilter eliminates it.

  13. Please use the “pin” on the google analytics script tag to let ScriptDeferFilter ignore the script tags. See my article how to use the “pin” feature.

  14. Hey it doesn't work for me…. I ve added ScriptDeferFilter to my app code folder… And in global ascx file it doesnt seems to work

  15. Asak,

    I have incorporated ur sample but the last part which is combining script files into one url I didnt understand. You have mentioned some configuration file changes required for combining scripts into one URL.

    can you please give me the working sample so I can incorporate the same.

  16. Hello, I really like this approach, you can look at my site how it works.

    Also I have adapted it to work with ASP NET MVC control, look at this code…

    protected override void OnActionExecuting(ActionExecutingContext filterContext)


    Response.Filter = new AdriaticGlobal.MVC.Utilty.ScriptDeferFilter(Response);



    just change in code HttpResponse to HttpResponseBase.

    You can see live example at: (beta site).

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