the short answer:
dial back \"quality\" settings, resolution, etc. and set the colour depth to 24/32 bits. set the stencil threshold to a smaller number.
the expensive answer:
a better graphics card, more ram, and faster computer.
the hard answer:
contribute to the cs dev team to optimize the code.
the simple answer:
(and somebody beat me too it, thanks Zebazar for reminding me)
update your graphics card drivers.
the non-trivial answer:
play with the many graphics settings until it works smooth. keep notes because \"inquiring minds want to know\"