There are two things that you can do to greatly reduce the lag that sometimes occurs in the World Editor. Both of these tricks work for both TFT and RoC.
First, to greatly reduce object editor lag, all you have to do is go to the Window drop down menu in the main window and deselect the Brush List.
The Brush List displays a list of brushes (units, doodads, etc.) at the lower left corner of the main World Editor window. Most people don't use the Brush List, but the World Editor has to load all the object data for it. When a value in the Object Editor is changed, the Brush List is refreshed, which causes the lag.
Secondly, if you have Windows XP or 2000, you can change the priority of the World Editor to make Windows run it faster. There are 2 modes for this, Above Normal and High. You should only do this if your computer is fast and if the World Editor lags.
Above Normal system requirements:
384 megs ram
1.7 ghz equivalent processor
32 meg video card
Fastest system requirements:
512 megs ram
2.4 ghz equivalent processor with a more than 266 mhz FSB
64 meg video card
To change the running priority:
Go to your Task Manager (control+alt+delete).
Click on the Processes tab
Right-click on worldedit.exe and select Set Priority.
Set it to Above Normal or High, depending on your system.
If your computer runs really choppy like this, immediately reset its priority to Normal or your computer will crash.
Do not ever set priority to Real-Time unless you have an uber fast computer, or your computer will become completely unresponsive until you close the World Editor!
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.
If you need to have other things running, such as an antivirus program or whatnot, you can reduce the other programs' priorities. Don't lower it too much or it won't run right. You probably shouldn't go down more than one level.