I use WindowsXP and it was lagged (P4 2.533, Geforce 5200 fx). Reason: compatability mode. I needed to switch to win98/me comp. to create a char, but the play was lagged! (1 fps, depending where you looked).

It was not my hardware, only when I looked in the direction of the temple. Frustum culling works Then I switched off compatability mode and now it works fine.

So for those lagged XP users:
  Try to disable compatability mode.