How to implement GZip compression in ASP.NET?

Here is the solution for css and javascript files. Add the following code to <system.webServer> inside your web.config file:

<configuration>
  ...
  <system.webserver>
     ...
      <httpCompression>
        <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll"/>
        <dynamicTypes>
          <add mimeType="text/*" enabled="true"/>
          <add mimeType="message/*" enabled="true"/>
          <add mimeType="application/javascript" enabled="true"/>
          <add mimeType="*/*" enabled="false"/>
        </dynamicTypes>
        <staticTypes>
          <add mimeType="text/*" enabled="true"/>
          <add mimeType="message/*" enabled="true"/>
          <add mimeType="application/javascript" enabled="true"/>
          <add mimeType="*/*" enabled="false"/>
        </staticTypes>
      </httpCompression>
      <urlCompression doStaticCompression="true" doDynamicCompression="true"/>
    ...
  </system.webserver>
  ...
<configuration>

Credit: How to GZip on ASP.NET and GoDaddy

this may be useful for you try it out, this accepts deflate and gzip compression.

    void Application_PreRequestHandlerExecute(object sender, EventArgs e)
    {
        HttpApplication app = sender as HttpApplication;
        string acceptEncoding = app.Request.Headers["Accept-Encoding"];
        Stream prevUncompressedStream = app.Response.Filter;

        if (app.Context.CurrentHandler == null)
            return;

        if (!(app.Context.CurrentHandler is System.Web.UI.Page ||
            app.Context.CurrentHandler.GetType().Name == "SyncSessionlessHandler") ||
            app.Request["HTTP_X_MICROSOFTAJAX"] != null)
            return;

        if (acceptEncoding == null || acceptEncoding.Length == 0)
            return;

        acceptEncoding = acceptEncoding.ToLower();

        if (acceptEncoding.Contains("deflate") || acceptEncoding == "*")
        {
            // deflate
            app.Response.Filter = new DeflateStream(prevUncompressedStream,
                CompressionMode.Compress);
            app.Response.AppendHeader("Content-Encoding", "deflate");
        }
        else if (acceptEncoding.Contains("gzip"))
        {
            // gzip
            app.Response.Filter = new GZipStream(prevUncompressedStream,
                CompressionMode.Compress);
            app.Response.AppendHeader("Content-Encoding", "gzip");
        }
    } 

For compressing JS & CSS files you actually have to handle that at the IIS level, since these files are rendered directly without the ASP.NET runtime.

You could make a JSX & CSSX extension mapping in IIS to the aspnet_isapi.dll and then take advantage of your zip code, but IIS is likely to do a better job of this for you.

The content-encoding header tells the browser that it needs to unzip the content before rendering. Some browsers are smart enough to figure this out anyway, based on the shape of the content, but it's better to just tell it.

The Accept-encoding cache setting is there so that a cached version of the gzipped content won't be sent to a browser that requested only text/html.