I had much lagging with my old P4 1.6Ghz on Tatoine too. Manaan was much better. The only thing you can do is try to lower the graphic settings as low as you can:
- Turn Shadows Off
- Turn Grass Off
- Click on Advanced Options
- Set Texture Quality to Low
- Set Anti-Aliasing to No
- Set Anisotropy to Off
- Set Frame Buffer Effects to Off
- Turn Soft Shadows Off
- Turn V-Sync Off
If you still can't make it but want to beat the track, then get my easy swoop racing mod:
http://www.pcgamemods.com/5287/) :D