The game certainly slows down the more you play it. My guess would be a memory use issue. If I notice the load times getting unusually long, like 10+ secs, and the frame rates start to drop I save the game and restart the xbox. That fixes the problem for me and my xbox slim and I only have to do it once a night. That fact that I have to do this is rediculous. This is the ONLY game I have ever had to restart my xbox to get reliable game play. If this game requires every bit of the processor and memory to play it reliably then it should come with an advisory, especially if as many on the forums have stated, it is actually breaking their xbox. The xbox does not have gauges like a car or system monitor software like a computer to tell you when it is being over used so the average person just has to guess when it is overheating.... being over used. Is my xbox crashing because its overheating or because the game is really buggy? Why would the average person think restarting their xbox a few times ruin it? If I buy a new car and restart it 30 times its gonna restart each time. If I buy a new computer and restart it 30 times its probably going to restart each time and if not, it certainly wont ruin it. So why should a new xbox give us that worry? Is it that big of a piece of junk? Honestly, having sent my xboxes in three times for the RROD, they certainly can be.
I guess the moral of the story is, if you are having this much trouble playing a game then take the game back. Its not worth ruining your xbox. Dont get me wrong, its a great game in theory, but in practice it is extremely buggy. I was one of the lucky ones that didnt have my save games corrupted. I did have to go back several times to a previous saved game simply due to bugs and quest issues, but I was able to finish it. I think if I had the issues some people have experienced I would have taken the game back and waited until next year when the GOTY edition came out and the bugs were gone. Its just a game.