Is stencil testing HW accelerated on Macs? After running my app with AGL_STENCIL_SIZE 1 it slows to a crawl.
You might want to check on this, but don’t you set the size to the bit depth you want? I think OpenGL supports upto 8-bit stencil buffers, so try other bit depths and see if they work any better. Also, what graphics card are you using?