I have been frustrated by the slow WiFi since february this year when I got my Retina, and now after seing this thread... have simply changed the Thunderbolt port in which I plug my external monitor in - and my WiFi is back to what it should be.
Of the two ports on the left, if I plug the monitor into the port closer to the power (left) then I have slow WiFi... if I unplug, and plug it into the one next to the USB (right) then I have normal WiFi.
Really pathetic, but this solved the slow WiFi for me.
Some Technical details if this may be of use to apple/others...
- I have a apple thunderbolt --> DVI adaptor (not a fake one)
- I use it to plug in a Samsung SyncMaster B2430
- I usually always have an external usb hub plugged in (extension is long, so the items I have plugged into the hub are far away, so no interference from those devices.
- I changed my router (bought new one), changed ports, changed all settings that were advised on other sites, and nothign worked... as each time my external monitor was plugged in and it was slow, and I did not realize then that when it was fast... that the external monitor was not plugged in :S
Thanks a lot. This all of a sudden started happening. The wifi router must have rebooted and switched chanels and all of a sudden my MBP wasn't getting any wifi. Strangely and stupidly, the wifi icon at the top of the screen still showed full connectivity but the thing wouldn't even ping the router more than 10% of the time. With the thunderbolt display disconnected sitting in the exact same spot it would work perfectly. I changed the channel of the WIFI and it works perfectly now! Thanks!