Over the last week or so, my game has gotten so sluggish that it was almost painful to play it. I had tried numerous things to try and speed it up and I finally found the correct answer so I thought I'd share it.
The first thing I did was go through my hard drive and delete any unnecessary files to free up some disk space, but that didn't seem to help. Then I tried Pam's suggestion and move all my screenshots to a different folder to try and free up some space in the game. That didn't seem to help either. I felt like there HAD to be something, somewhere that was slowing down my game, so I started looking in all the game files. I knew I had not installed any new CC or any mods so it couldn't be that.
I came across the folder called DCbackup located in My Documents--> Electronic Arts--> The Sims 3--> DCbackup. I looked at those files and realized that they were just a bunch of backup files of all the package files ever used in the game. I did some checking to make sure it was ok to delete those and found out that it was fine. Once any CC is installed, it's there in your game, so you really don't need backups of it. But if it will make you feel better, you can always move all of those files to another folder somewhere other than your Sims 3 folder. But the best thing to do is just to delete all of those files in the DCbackup folder. In fact, it's probably a good idea to clean it out weekly or monthly....depending on how often you play. If you have any premium content installed from the Sims 3 store, do not delete the ccmerged.package file within the DCBackup folder. You will need to reinstall the items for them to once again be able to work properly if you delete this file
Once I deleted all of those files, I was amazed at how much better my game played! It's like brand new again! I think that folder basically acts as a cache to hold all the items that are used in the game for easier access. The problem is, just like with web browsing, once you get too much junk in there, it just slows everything down.
Edited to add the following from mtglady:
You can also delete your compositorCache file, CASPartCache, scriptCache, and your simCompositorCache file. Just delete them all as they will regenerate when you start a new game. They can be found in your 'Your Name' > Documents > Electronic Arts > The Sims 3 folder. They are standalone files so if you sort the Sims 3 folder by name, they will appear toward the bottom. If you delete the socialCache file, there is a good possibility you may lose all progress towards achievements and showtime Simport stamps so be careful about getting rid of this file.
And please do as Pam stated, restart your computer, close all unnecessary background files and try loading your game again. Hopefully this will cure your problem for awhile.
I delete these two files every evening when I am done playing and my DCBackup files monthly.
Note: For Windows Vista and Windows 7, look in Documents, not My Documents.
Edited by Pam on March 30, 2012 (added more cache files that can be deleted.)
Edited by Ricalynn on November 2, 2012 to include information about the ccmerged file