I'm having the same problem, and it's only been getting worse over time... I'm now up to 2:40 minutes loading time between lots.
Things I have tried and did not work:
- Clearing my cache (which never has more than 2 files)
- Clearing all unplayed sims (Which I've been doing since day 1 anyways.)
- Clearing my Library/tray folder
- Playing the sims offline
- Playing with Origin shut down
Out of curiosity, and an attempt to reduce the size of my save file I also:
- Deleted the entire existing population of sims (including both played and unplayed) with the exception of 1 family (containing 2 adult sims and a baby)
- Bulldozed all lots with exception on 1 (Which I saved for that one family.)
None of this worked, and did not reduce the size of my save file.
When I start a new game the loading time is perfect (5 seconds max), which is what I experienced when I first started playing my current saved game.
* I just wanted to add the rest of the game works perfectly for me, no lag at all. It's just the loading screen bewtween lots that's giving me issues. My main conceren is that it keeps getting longer overtime with no end in sight.
ETA: My DxDiag
http://pastebin.com/raw.php?i=LZ2Xpn5K