It depends on the size of real numbers wheather truncating makes sense, and also algebraic opeartion scale, accumulating loss of precision when consequently passing through them.

Do it twice or three times and you're done for.

That sounds wrong to me. Sweeping a few capsules through some collision geometry shouldn't take any perceptible time. Have you profiled this, and seen what's going on? My guess is that it's doing a whole lot more work than necessary, and perhaps there'…