You can use onMouseMove event on form for the tile-painting, then it doesn't matter if you are using 16+ interval. Actually that could be the reason for high cpu use in the first place. Game template-based programs run same speed on all machines, but it's important that you put game mechanics into GameLoop, and rendering into Rendering procedure. If you would mix game mechanics with rendering code, frameskipping could mess up the sync.

Also keep in mind that you have another thread specifically for the GTA editor, this is about the engine alone.