[Chris]

When you run the game in Direct3D along with other processes in the background, it drops your framerate terribly. 640x480 is the alternative.

Also, maphack slows performance in fullscreen mode.

To run the game in Windows mode:
Simply open up your shortcut, and add this to the Target:

"C:/Program Files/Diablo II/game.exe" -w

I do recommend you run in windows mode. It makes your framerate 60-120 most of the time.