Bump. No one know? Any guesses?
No one really knows, everyone has jumped on the save file size band wagon, because it was the only correlation found. I personally am not convinced of this. The file sizes for when the lag sets in are in consistent. 5MB, then 6MB, then 7MB, then 8MB, it keeps going like that. It is however completely possible it is related to file size.
Some have said memory leaks and caching issues, which kind of ties into the the save file size theory as well.
Another possibility, is quests or items that are glitched causing this. I only say that because there was an Xbox poster who posted a lag issue in the Xbox section, and it turned out to be a glitched key they were carrying, they cured it by dropping the item. Everyone with issues should be comparing items and quests we've taken, and locations traveled to just to make sure.
There's a seemingly less likely possibility that it's a massive batch of faulty game discs, probably something to do with the manufacturing of the game discs.
It's pretty much up in the air.