I have exactly the same problem, with both WinSPWW2 and WinSPMBT. Actually I only now noticed that the lag only appears in full screen mode and not in windowed mode.
I have 2GHz A64, 1.5GB RAM and Radeon 9800 Pro (128MB RAM) and I run Windows XP, so the problem is software based. Please ask for any additional information you think might help track down this issue. I would love to play this game full screen without this terrible lag
