I have a 100 mbit unlimited internet channel. When I lauch speedtest from my iPhone 5 it shows 42 mbits of wifi speed. All pages open very fast, but when I try to download something from iTunes and App Store it can take many minutes. Now I'm trying to download on album from music store, and it shows that download spped is about 100 kbits. I was updating tweetbot recently and it took 12 minutes to do that. Same speed probems are with 3G connection.
My iPhone 4S on the same network with iOS 5 don't have such problems.