Enabling dynamic and static compression in IIS7 is easy, well-documented, and beneficial for performance in almost every scenario.
However, compression of JSON data is not enabled by default.
Here's how to enable it:c:\windows\system32\inetsrv\appcmd.exe set config -section:system.webServer/httpCompression /+"dynamicTypes.[mimeType='application/json',enabled='True']" /commit:apphost
(Your drive letter and windows folder may be different than my example above.)
Then recycle your app pools ... and you're done.