I have the exact problem however it only happens at home WiFi (airport express) but not at office. I don't know what type of WiFi AP they use at the office but definately not an Apple one.
I've also tried streaming the same videos with my iPad 2 and It was also quite fast.
Both the iPhone 5, iPad 2, iPhone 4 and Airport Express are using the latest firmwares.
Did anyone found a way to resolve this?