Review login-in items
If you added login-in items in System Preferences -> Login Items, review them. Some of them you may not need anymore.
Disable Disk Journaling
Journaling may make your filesystem less prone to errors, but it does so at a large performance decrease. If journaling was enabled before, you can disable it in the Terminal by typing sudo diskutil disableJournal /
Don't use a login window on a single-user system
Disabling the login window if enabled (System Preferences -> Accounts -> Log in Automatically) will make booting up faster (not just counting the time needed to type the password.)
Don't use "suck-in" minimizing
"suck-in" minimizing can be enabled in TinkerTool, but uses more proccessing time than "scale" or "genie".
Disable menu fadeouts
Download and install FruitMenu and go to System Preferences -> FruitMenu and check "Disable Animated Menu Fadeouts." This will make menus snap instead of fading in and out when you use them.
Disable window shadows
Use ShadowKiller (free) or WindowShade X (shareware), both from Unsanity, to disable windows shadows. This might slightly increase performance on Quartz Extreme computers and drastically on regular Quartz computers.
Speed up 'sudo' operations
If you missed this hint, use it to speed up 'sudo' operations by about a second.
[robg adds: Some of these have been published before in other hints; if you're interested in more tips for affecting the speed of OS X, check out this hint from February of 2002. Some of the info may be a bit dated, but there's still some good advice there as well. Personally, I'm more than happy with the speed of OS X on both our G3/500 iBook and the new 12" G4/867 PowerBook, but if you're not, these hints can help.]

