Flipz (of Fresh ROMs) has come through again, this time with a solution to slow data speeds. It seems the problem lies with PRI - as Flipz puts it, “... a connection between your phone and the radio... responsible for carrying both voice and data.” The EVO shipped with PRI version 1.40, but rooting the phone downgrades to 1.34 - which is evidently less efficient.
The fix isn’t exactly easy, but it’s nothing too tricky, either. Users who have experience in the depths of their phone (read: recovery, logistics, etc.) should be just fine, as should less-experienced users who follow the directions closely.