Prevent CPU gobbling in In.UpdatePhysics()

I was horrified to see the library eating up 100% of one of my CPU cores. I ran a profiler on my code, and 99% of the CPU time was spent in the `Stopwatch.ElapsedMilliseconds()` function found inside of `In.UpdatePhysics()`. I've verified that the physics are still accurately computed, but at practically no CPU cost.