http://www.chromium.org/spdy/spdy-whitepaper has data on improvements in lab tests. http://googlecode.blogspot.com/2012/01/making-web-speedier-a... has a blurb where Google announces that they've made search (already highly optimized) faster with SPDY. This is a fascinating result, because this result was obtained after Google Search switched to using HTTPS, which typically makes websites slower, but in Google's case, made it faster because of SPDY. Note that Twitter and Facebook have also adopted SPDY. I'd be rather skeptical that Google, Twitter, and Facebook would all switch to SPDY if there weren't real world benefits.