I am having an issue with this as well. If I turn off cell data and put the phone in airplane mode, I am getting great speed and connectivity over wifi.
Wifi on/cell data off/airplane mode on - 31ms ping ~30Mb down ~10Mb up
Wifi on/cell data off/airplane mode off - 336ms ping, .17Mb down .14Mbup
Wifi on/cell data on/airplane mode off - Unable to complete, Speedtest reports Network communication issues.