It is rare that a game played offline would experience lag, but Bethesda has found a way to do it in Skyrim. Apparently on PS3 copies of the game, if your save file exceeds 5-6 mb on the PS3 hard drive, it slows the game to a crawl. This bug keeps you from interacting with NPCs or completing quests. Ex OPM UK, Editor Tim Clark has a experienced this himself and had this to say.
So it would appear that the issue stems from the size of a save file. A save file will continue to increase in size as a player spends more time in the world completing quests and growing as a character. The work around appears to be turning off the auto save feature, but I would think that would be worse. I can only imagine forgetting to manually save and losing hours of content. Bethesda tweets they are "continuing to work on an update for all platforms to address issues".
Never have I been so happy to have an Xbox 360 copy in the office. Also, don't forget to check out our review of Skyrim.