It's a caching issue, it affects all computers and isn't directly dependant on CPU or RAM (although having bad either will make an existing problem worse). The problem gets worse if you look at a large number of sims, or dress sims very differently. Looking at other sims causes the game to have to cache their appearances, and dressing sims very differently causes the cache to miss a lot because the item you want to see isn't in the cache. A slow or fragmented hard drive will worsen cache accessing speeds. After about 2 GBs of RAMs, more RAMs won't help because the game seems to actively resist using more RAMs, dumping things to disk cache even when plenty of RAMs are available, to avoid hitting the 2 GB crash point for 32bit. So even if you have more RAMs, a 32bit program can't normally use more than 2 GB, period, and the game will resist running into this limit. If you have a metric fuckton of RAMs and want to REALLY make things go fast, save everything onto a RAMdisk, so when the game tries to dump to cache, it will still be using RAMs and not disk. This way your 'Disk' is also as fast as RAMs are and you will notice much less thrashing.