It could be the stencil buffer -- I know that older cards don't like stencil buffers when running in 16 bit mode, and as such they may fall back to software rendering. These cards much prefer 32 bit when dealing with stencil buffers.

What's the output of "glGetString(GL_VENDOR);"? If it's Microsoft then you've fallen back to software rendering instead of hardware for some reason (not necessarily due to the stencil buffer, but there'd definitely be an issue somewhere).