Badapple, that shouldn't have any effect on the speed.

CodeMaster, that is a general problem with the way stencil shadows work and nothing specifically related to gamestudio. The only solution is to have all objects casting shadows, as that will of course hide the "wrong" shadow in the shadow.

Lite-C Tut. 1 Lite-C Tut. 2 Lite-C Tut. 3 (en)
Shader Tutorial (ger)

http://www.slindev.com