People need to realize that the Cell may be very fast for some things, but it is effectively 1/3rd as fast for other tasks. This frame rate issue is almost certainly tied to the 360's 3 core CPU vs the Cell.
The biggest reason framerate is an issue is because of input responsiveness. Consider the way engines render things:
One frame is currently displayed. A second frame is rendering. At this point, a user is responding to the frame that is displayed (or the frame before). Obviously, the game must calculate what happened due to user input, must respond to it, and at the very soonest moment it will show it on the frame after the next frame is displayed since it already has the information for the being rendered frame. Consider a very simple engine at 30fps:
Current frame sticks around for 1/30rd of a second.
Next frame will stick around for 1/30rd of a second.
Third frame (earliest possible with the user input) is around for 1/30rd of a second. The response from user input will always be 2-3 frames out. This could be mitigated if you have more CPU cores and you could have it be 1-2 frames out instead.
This means user input will have a delay of 1/15th of a second to 1/10th of a second -- 100ms. This is the absolute best case. If the game was doing something like triple buffering to improve the framerate, it would be significantly worse. If the game is doing vsync, it may not be able to render frames at that speed.
Things get more complicated real fast. Assume the game is set to output at 720p/60 but is strugglin to render at 30fps and vsync is enabled. This means it may be rendering 20fps (due to vsync). Your latency goes up to 1/10th of a second in the best case and less than 1/7th of a second in the worst case. This feels *very* laggy.
There are tricks you can do to attempt to increase the framerate, but many of them cost responsiveness for user input. If the PS3's user input is really that laggy, the game would be mostly useless to core gamers who are interested in a top-end game experience. The 360's framerate running 60fps is nice for visual reasons, but also control latency reasons.
