Some of the problem may be your video processor and how it handles the sprites. Especially if you're having slowdown problems in the forests and busy towns. I had no trouble with the walking speed in general on my 24" iMac. No slowdowns. Occasionally restarting the machine to clear stuff clogging the memory (especially if you're running OS 10.6.3, which appears to have an annoying memory leak in the video processes) may also help.
Kill 'em all, let the sysadmin sort 'em out.