Dynamic content compression does not work by default in asp.net for ‘application/json’ mime types, and given that we’re doing more mobile apps and client side data requests this can make quite a difference to application performance. In flymapper my json response went from 0.4mb to 0.04mb.
http://www.hanselman.com/blog/EnablingDynamicCompressionGzipDeflateForWCFDataFeedsODataAndOtherCustomServicesInIIS7.aspx