I previously posted that changing MTU to 1300 fixed the problem for me, but I changed BACK to MTU=1500 and my system is still "fixed": wi-fi performance is at same level as it was pre-10.8 upgrade. (My original problem was that wi-fi speed drastically decreased after 10.8 upgrade. Handheld devices on same wi-fi network worked fine.)
I got suspicious when I read someone's post that said that they switched to WEP from WPA2 and that "fixed" their problem. I have been using WPA2 all along.
I now suspect that something in the process of MAKING a change causes some sort of reset and that is the actual fix. But not all changes that I tried worked.
Somewhere along this process I stumbled on a "fix" for my system:
1. A few days ago cycled wi-fi off/on (that worked for someone else in another thread). No joy. Didn't work.
2. A few days ago changed MTU from 1500 to 1453. No joy.
3. Turned wi-fi off. Used ethernet hardline to wireless router for a few days. Everything fine - bypassed problem.
4. Today, created new wi-fi network. Didn't end up using it. Using original wi-fi network, changed MTU from 1453 to 1300. Speedtest nominal. "Fixed" problem.
5. Less than 1/2 hour later changed MTU from 1300 back to 1500 (which was working fine pre-10.8 upgrade). Speedtest nominal. System still "fixed".
Again, I suspect some sort of reset occurred causing internal wi-fi hardware, driver, and osx 10.8 to play nice with each other. Someone smarter than me will have to figure this out and explain it.