Comment 50 for bug 124159

I'm having this troubles, too, using bcm43xx on a 4318 chip. NM tries to connect with 24M, which causes poor performance. In feisty the default value was 11M, AFAIK due to restrictions in the kernel-driver, which prevented higher rates. Now these seem to be no longer present, though higher connection rates are still unstable, at least with the BCM4318 chip.

After setting to 11M manually (home network) or 6M (University), everything is ok again. To use the workaround together with NM, you can add a script to /etc/NetworkManager/dispatcher.d, containing the iwconfig stuff.