I coded an opengl scene where i draw a floor and a ball. I want the floor to reflect the ball, but i want no artifacts, so i used the stencil buffer. However, I managed to render it correctly, but it is really SLOW. A Direct3D sample that does the same thing is moving pretty fast. My card is a nVidia Riva Vanta /w 16MB. What is the problem?

I guess, you works in 16bit color mode.
But TNT has h/w support for stencil buffer only when in 32bit.