I made a physics based Asteroid game with JBox2D, and had physics running in one thread and rendering in another. Worked wonders with antialiasing on dual cores, lagged to death on singles. Well, that was back when single cores actually still existed.
I had a similar experience but on a multicore and switching from one thread to two. Sometimes it wouldn't leave time to do a good A-star search even though it didn't lag. I wanted to do searches on multiple threads anyway, but I couldn't figure out why it did that while still having time for multiple rendering updates per physics update.
(Edit) What does this line do?