Good discussion on this at xda... basically the majority agree that Root with nand unlock is changing the PRI to an older version and many people have seen their speeds decrease by as much as 50% by doing doing this. The max download I can get after rooting has only been ~70-80 kbs. I would sugest NOT to do a full root if you use your 3g a lot until this issue is resolved! As of right now there is no way to revert back to the newer PRI versions that ship with stock...
Here is the discussion at xda:
http://forum.xda-developers.com/showthread.php?t=712824