WIFI is a funny thing. And USB WIFI has always been a hit or miss thing on speeds and connectivity.
Just cause you have full bars doesn't mean you will get good speed. There could be underlying interference causing the problem.
Also, as stated, drivers may not be up to par as everything should still be considered beta, and drivers should also be till well into dec/jan/feb (some 3rd parties even longer).
I would suggest looking for Win 7 7264, search for the hashes here on the forum and elsewhere and make sure they match. 7264 is the last closest release to RTM that I know of.