How to enable GZIP compression on a EPiServer 7 site on IIS 7.5

First you must add the Dynamic compression in your IIS, go to server manager and add Roles and select the Dynamic Compression.

Then just add the following into your web.config in the webServer node:

 <httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files">  
          <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="application/x-javascript" enabled="true" />  
             <add mimeType="application/javascript; charset=utf-8" 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="application/x-javascript" enabled="true" />  
             <add mimeType="application/javascript; charset=utf-8" enabled="true" />  
             <add mimeType="*/*" enabled="false" />  
          </staticTypes>  
       </httpCompression>  
       <urlCompression doStaticCompression="true" doDynamicCompression="true" />  

Then you will se a good A score on your YSlow tests!