Not sure if this will help anyone else, but I've found that the iPhone/iPad in particular have a very sensitive-to-interference WiFI network adaptor. I've been struggling with this for a while, but have solved the problem for me, anyway. I have 2.4 Ghz and 5 GHz being broadcast over the same SSID. It didn't matter which bands the iPhone grabbed, I was getting awful performance (single digit Mbps down and up). I know all 2.4 Ghz bands are saturated around me, but I was puzzled on the 5 Ghz. In any case, I changed the 5 GHz channel from the default of 32 (5180 MHz) to 149 (5745 MHz) and voila, the iPhone's WiFi is blazing fast.
This may also explain why others experience it and others don't. Different devices respond to RF / WiFI interfence (this increasingly becoming an issue) differently, it all depends on the network interface adaptor. Some are more resilient than others.