We bought iPhone 5 and a 4S about a month ago. I also have a 3rd generation iPad.
All 3 devices have very poor data rates on wifi, typically 1-2 Mbps. I've also read and tried just about everything, including network reset on devices, changing wifi encryption, changed to brand new wireless n router, etc.
Other wireless devices (non-iOS) consistently get 30-33 Mbps using speedtest.net. On very rare occasion I can get 5-10 Mbps on the iphone, but it doesn't last long. In the Apple store there is no problem with their wifi, then again that signal is not encrypted.
Because I've seen all 3 iOS devices at times get higher data rates, (a rare occurrence), and because all 3 devices behave similarly, this leads me to believe the cause is iOS issue that has a problem on a wifi network with other devices with encryption enabled.
Patiently waiting for Apple to fix with next iOS update. (by the way, my iPad was fine before the latest releases of iOS).