And a memory leak I believe. A friend of mine would always have a slowdown over time and eventually a memory flow overload error forcing a hard restart. Although it takes around 2hrs of gameplay for it to actually take its toll it still crashes on multiple systems of his.
About the framerate though, depends on the system. He has a powerhouse laptop that runs super high resolutions at insane frames. It's like pure bliss and at higher than HD resolution on the 17" screen makes Halo 3 look like complete ass. One thing many console gamers forget is cutting-edge PC games are built to run at their peak years later once hardware finally catches up, and at that point there's another couple of killers.