For a while I had terrible (in the 800msec - 4000msec range) RTT pinging an AP right next to me, running:

    sudo iw dev wlan0 set power_save off

did the trick for me (issued after every boot or through an UDev rule such as https://raw.github.com/lvillani/ansible-playbooks/9dc40c75cf...)

I didn't actively do anything to fix WiFi. I just upgraded to the latest release of Ubuntu and enabled backports in the repository settings to stay current (but not in bleeding-edge dev mode, no thanks I gotta work). The driver seemed to crash on coming out of suspend/hibernate/whatever sleep state it was in. This kept generating automatic error reports that were phoned home. A few weeks ago this stopped happening so either Ubuntu or kernel Wifi devs or somebody somewhere has gotten a fix in. Let's hope it stays that way. I really didn't dig into it.