It's been a while since I played, but I remember it being an issue with the asset cache folder which is supposed to get purged on restart. The folder holds onto things like player customizations and stuff like that. Once it gets to a certain size I remember getting some pretty bad performance drops. Most people got around it be putting the folder outside of their primary drive via symbolic link. I'm sure the original thread it came from is still on the forums somewhere. I would imagine once the engine gets updated that shouldn't be necessary anymore.