do not have specific info about your adapter, however, as you probably know, wireless adapter use and adaptive technology to change the "on air" speed using some algorithm.
the only suggestions, right now are:
- check the real rate settings of your device using iwconfig, eventually you can force some fixed value, i.e. 54 Mb/s.
- check and report back what kernel are you using, eventually there could have been some improvements on recent kernels/rt2870 drivers
- you don't tell us how you measure the "speeds" you are reporting, are the wireless speed ? or the overall speed (wireless + ISP) ?
as of recent kernels (2.6.38.x) I do not see an rt2870 module available, only rt2870sta.