My problem is similar. I have a MacBook Pro 2011 running Mountain Lion and a 23" Acer Monitor. I have WiFi when connected through Thunderbolt port, yet it is extremely slow. 1.5 Mbs download when connect to monitor, 10.5 when just using the laptop normally. Interestingly, upload speeds are not affected by the Thunderbolt monitor.
Seems to be an OS bug, though. Nothing seems to work.
Any info would be greatly appreciated, however.
UPDATE: I have solved my problem, and want to share this info in the event it helps anyone.
When my MBP is closed (clamshell) and connected to external monitor through thunderbolt port, i get slow Wifi. If I open the laptop, however, Wifi is normal. I suppose this isn't technically a solution, but rather more of a work around. However, the end result is my stuff works as it should now.
Hopefully, Apple will fix this weird bug soon.
FYI, I had some trouble seeing everything on my display with the MBP open. To solve this, I went into display settings on the MBP and clicked "Mirror Displays."
I have just testing using a different setup and am able to maintain both wifi and my external monitor displaed.
I tried using the same external monitor using a DVI-HDMI adapter. While connecting my monitor through the HDMI, I am able to maintain wifi conenction and monitor use.
I called the Apple Support that suggested that this might have been the issue of interference with the adapter not have shileding for interference near the Thunderbolt connection.
I am experiencing something similar.
MacBook Pro Retina. Connect to external monitor via the Thunderbolt DVI adapter. Wifi speeds drop from 10 mbps to under 1 mbps.
Wifi works just fine on iPad and other devices.
Disconnect the TB DVI Adapter and the Wifi speed immediately jumps back to normal. No reboot necessary. At least that made it easy to determine the source of the problem.
Am going to try why savvypavvy did and run the monitor through HDMI > DVI Adapter tomorrow and see what happens. Pretty lame if it is a shielding issue.
I was having this same problem.
Macbook Wifi worked great when standing alone.
When connecting to my Dell external monitor via Miniport or HDM > DVI adapter, it immediately cut out. Unplugged it, and it worked great.
Tried using an HDMI > DVI adapter with extra long cord to see if there was an issue with poorly shielded adapters (cord moved the adapter part far away from the mac itself). Worked great when on my lap, but again the Wifi hung when I put it on the desk.
At that point, I figured it wasn't the adapter shielding, or a software problem. It was from the monitor itself.
Solution was to change the channel on my router. It was set to automatic. I manually cycled through the different channels until I found one that avoided the same frequency as the monitor. Problem solved.
Also heard that baby monitors can be the source of interference.
I had the same problem conecting an old HP2035 monitor. I cheked wich WIFI chanel my router was on (7) and changed it to 13. and voila the monitor works fine on a thunderbolt to PCI conector and the WIFI is back to its full speed. I don't think its a connector sheilding issue but simple monitor interference with chanel 7.
I have the MacBook Pro with Retina, and I had the same wifi issue with both Thunderbolt connections with my DVI-Thunderbolt adapter. I have not tried the wifi channel change that legli suggestioned. I using a different DVI-HDMI, and I am able to consistently maintain wifi speeds.
That said for the HDMI connection, there have been times when my monitor goes to black for a second or two, even if I am typing or doing some active to prevent the monitor from going to sleep. It is intermitent.
My brother who also has a Retina display, had the same issue of an external monitor through a different DVI-Thunderbolt adapter.
Very similar issue here:
Brand new MBP Retina + DELL 2007 FP monitor, connected via a ~ 2 year old Apple Mini DisplayPort to DVI adapter. Extremely slow WiFi traffic or even problems connecting to WiFi. No matter which of the two Thunderbolt ports, no matter wether the laptop is open or closed.
Everything is working fine as soon as the DELL monitor gets unplugged.
I also went into my router settings, and changed the channel to 11, and it solved the problem. I have a mid-2012 Retina MBP running Mountain Lion 10.8.2, and as an artist I depend on my Wacom Cintiq to make a living. I was extremely frustrated upon seeing my wifi connectivity disappear when I plugged into either of the thunderbolt plugs. Hope this helps!