Recently my Finder's contextual menu (right-click) menu has been taking an age (five seconds or so) to pop-up. Logging out and in fixes it, but not for long.
So I tried removing various items from ~/Library -> Contextual Menu Items and /System -> Library -> Contextual Menu Items and restarting Finder. It turns out that the culprit was FolderActionsMenu.plugin, in the System-level Contextual Menu Items folder. I have no idea why it makes things so slow (the Folder Actions Setup dialog is really unresponsive, which may have something to do with it), but moving it out of the Contextual Menu Items folder (as root or sudo) has made everything better.
You can still configure Folder Actions using the scripts in the Script menu's Folder Actions folder.
[robg adds: I haven't noticed a slowdown on either of my machines. If you're going to try this fix, don't delete the file, just relocate it elsewhere -- you may wish to put it back at some point.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060307035346486