90fps means 1000/90=11.1ms per frame. If your CPU finishes the physics in 3ms, your GPU has 8.1ms to render the scene to not exceed the budget for 90fps. If you have a slow CPU that finishes in 6ms, your GPU must finish all it's work in 5.1ms. So if you have a 7800x3d or 9800x3d with 6000MT/s...