I'm getting massive framerate hit/stutter/lag in the tundra Whiterun hold. I'm 25 hours in and level 16. What's going on?

How big has your savefile gotten in kb?
Also, check a few of the other threads here, sounds like you're hitting the point a lot of us have.

You need to check your gamesave file. It's most likely, although we need Bethesda confirmation, there is another data leak issue on save game files that is sending them to unstable levels of size.

It's easily the biggest issue on PS3, so I would expect some comment from Beth this week. It's still my only issue. I have no problems with anything else, including the sound that appears absolutely fine.

Same problem. Please Beth don't force me into buying Mw3

Anybody else? It was happening after a 12 hour play session. I haven't played today yet. Would a long game session be the cause? I haven't fought any Dragons yet and haven't done any Guild quests. Only miscellaneous quests.