Allister posted how to do prevent the game slowing down after about an hour, until a patch comes out.

1. Save game and reload. (Fixes the problem only for about an hour and then you are back where you started)

2. Go to www.tweakfiles.com. From there download a little program called Cacheman, set the memory to "Power User", and it will have the game a) speed up load times, and b) put off memory leak problems by nearly doubling the amount of time you can play before unloading data from the cache.

3. Do a full install. (For some reason, doing a full install does everything solution 2 does, but permanently.)

Also, some people have reported that running with the desktop set to 16 bits color seems to solve the problem for them. This doesn't seem to work for everyone though.