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