You can't stop it from lagging. This has to do with the type of video card you're using. Does it have "M" extension in its name? (if it's a laptop then it most definitely does; example - GeForce GT540M)
TopoGun uses some OpenGL extension to support heavy meshes, usually the mobile versions of even the latest video cards don't fully support those.
Guess I misread your post. If it worked fine earlier, then you could try testing combinations of PixelShade, RenderBoost or Antialising options in the Preferences window. Start with disabling Antialising. It is also a good idea to check how much memory Topogun is consuming.
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