Web API Gzip not being applied

Question

I have added the web.config entry to enable gzip compression based on this S/O answer Enable IIS7 gzip.

I then checked the Chrome Developer window while loading an ASPX page and saw the header in the response:

Cache-Control:private
Content-Encoding:gzip
Content-Length:3669
Content-Type:text/html; charset=utf-8
Date:Wed, 04 Mar 2015 00:46:05 GMT
Server:Microsoft-IIS/7.5
Vary:Accept-Encoding
X-AspNet-Version:4.0.30319
X-Powered-By:ASP.NET

So that means it's "working", correct? But when looking for that header when making a Web API call, it is not present:

Cache-Control:no-cache
Content-Length:551
Content-Type:application/json; charset=utf-8
Date:Wed, 04 Mar 2015 00:53:05 GMT
Expires:-1
Pragma:no-cache
Server:Microsoft-IIS/7.5
X-AspNet-Version:4.0.30319
X-Powered-By:ASP.NET

I have tried all kinds of different configurations (starting with the one recommended in the linked S/O answer above). Finally, in an act of desperation, I set it to this, which I thought would make it try to compress all requests (everything except */* commented out):

  <httpCompression directory="%SystemDrive%\inetpub\temp\IIS Temporary Compressed Files">
    <scheme name="gzip" dll="%Windir%\system32\inetsrv\gzip.dll"/>
    <dynamicTypes>
      <add mimeType="*/*" enabled="true"/>
      <!--<add mimeType="text/*" enabled="true"/>
      <add mimeType="message/*" enabled="true"/>
      <add mimeType="application/javascript" enabled="true"/>
      <add mimeType="application/json" enabled="true"/>-->
      <!--<add mimeType="*/*" enabled="false"/> -->
    </dynamicTypes>
    <staticTypes>
      <add mimeType="*/*" enabled="true"/>
      <!--<add mimeType="text/*" enabled="true"/>
      <add mimeType="message/*" enabled="true"/>
      <add mimeType="application/javascript" enabled="true"/>
      <add mimeType="application/json" enabled="true"/>-->
      <!-- add mimeType="*/*" enabled="false"/>-->
    </staticTypes>
  </httpCompression>
  <urlCompression doStaticCompression="true" doDynamicCompression="true"/>

What can be preventing the GZIP from being applied to my Web API methods?

Update

I have since tried both the NuGet Web API Compression package, and editing the applicationHost.config in both IIS Express 8.0 (Visual Studio) and a locally-installed IIS 7.5. All have yielded the same results: requests for other mime types like text/* work, but application/json refuses to be gzipped.

Answer 1

Is the WebAPI behind a Firewall, Web Proxy, Virus Protection Suite? As mentioned in Even Faster Web Sites: Performance Best Practices for Web Developers By Steve Souders This could be stripping out the headers.