This morning, I got 10-15+ Mbps on both iPhone 5 and iPad (3rd generation). This was very early morning (e.g. minimal network traffic, interference, etc.) Later same morning, both devices drop to 1-3 Mbps, I assume because there is more wifi activity in and around my house.
Late morning I changed my router channel from Auto to channel 1 as recommended by Lawrence.
Now getting 25+ Mbps on iPad. Interesting fact on the iPhone5, I get a range of 1 Mbps to 20 Mbps. Why such range in results when I run speedtest within minutes of multiple tests?.
My non-technical conclusion so far: Since I have seen the iPhone 5 hit 20 Mbps at times, (though mostly 1-3 Mbps), this to me says its not a hardware issue. The variability on the wifi network is the mystery. Note all other devices (non-iOS) routinely get 20-30 Mbps on my wifi (my cable package is 30 Mbps into the house)
Perhaps iPhone 5 (and our 4s) have less "powerful" wifi capability when compared to iPad, and thus fall to the back of the bandwidth line when the network is busy?