There are 2 modes for this, enhanced and fastest. You should only do this if your computer is fast, but WE lags.
Enhanced system requirements:
384megs+ ram
1.7ghz+ equivalent processor
32meg+ video card
Go to your task manager (Control+Alt+Delete) and click on the processes tab and find worldedit.exe, righ click on it and go down to Set Priority and set it to Above Normal and no higher than that.
Note, if your computer runs really choppy like this, immediately reset it's priority to Normal or you're computer will crash.
My computer is really good, but it won't even run on "fastest" properly, but enhanced is a big difference.
Fastest system requirements:
512megs+ ram
2.4ghz+ equivalent processor w/ more than a 266 FSB
64meg+ video card
Repeat the same steps as above, except set the priority to High.
Either you have a fast computer, or you want to see how far you can push your computer, or Above Normal doesn't make enough of a difference, set it to high. But if it starts to become unstable, set it down to Normal.
If you try to use any other programs while you have the priority changed, your computer will be slow. It just means your computer wants to play with WE, and will leave other programs outside, making them slower. Hence the priority (sorry for the bad comparison :D)
EDIT: Another thing you can do, if you need to have other programs running, such as norton antivirus and whatnot, you can reduce it's process priority, I wouldn't lower it too much otherwise it won't run right. But you can lower programs you aren't using, I suggest by only 1 level.
NOTE: DO NOT EVER EVER SET PRIORITY TO REAL-TIME UNLESS YOU HAVE AN UBER FAST COMPUTER, OR IT WILL BECOME INSTANTLY UNRESPONSIVE UNTIL YOU CLOSE WE!! | 
