I noticed the same problem with ath9k on an AR9280 rev.2 chipset.
I tried compat-wireless for 2.6.38 but that didn't helped much (fixed dma errors and chip lockup issues, but speeds were still bad).
Using today's compat-wireless snapshot (compat-wireless-2011-03-20) seems to restore normal traffic speeds, Hurrah.
I noticed the same problem with ath9k on an AR9280 rev.2 chipset.
I tried compat-wireless for 2.6.38 but that didn't helped much (fixed dma errors and chip lockup issues, but speeds were still bad).
Using today's compat-wireless snapshot (compat- wireless- 2011-03- 20) seems to restore normal traffic speeds, Hurrah.