They both coincide with each other. If your Graphics processor(s) cannot keep up, your frame rate suffers. If your CPU is insufficient or gets loaded due to Physics, your frame rate suffers. If both pieces of hardware are quick and are more than enough for the game, your frame rate improves.
Then of course, you bump up on the issue of optimization. Games that are poorly optimized or built will suffer no matter how fast of a PC you have.