You haven't mentioned all what you've tried to troubleshoot, so please forgive any repeated tips.
When my game is running slow (no matter which save I try to play), I look first at what I've downloaded and installed recently. That means, custom content and store content.
I go through the cc and mods I use to make sure all is okay. Deleting the game cache as I test them.
Sims 3 - How-To: Fix Problems with Mods/CC
https://www.carls-sims-4-guide.com/forum/index.php?topic=31759.msg506010#msg506010This also applies to Sims 3 Store content as well to some extent. To much of it can cause slow loading as well. -- Any added content can put more strain on loading times. This is where we have to find our happy medium as I call it.
Check my Sim's personal and family inventories. As too much stuff in the inventories and on their lot (furniture/decor/landscaping) can cause lag.
Check my graphics and gameplay options to see if I need to change anything.
Sims 3 Options Overview
https://www.carls-sims-4-guide.com/forum/index.php?topic=27533.msg455148#msg455148Clean out the folders in my Sims 3 folder. Delete or backup old save games, move the screenshots, video, library, downloads and exports folder to the desktop or a usb key for safe keeping.
I've read where the town you play in also can cause lag as well. Larger towns can be fairly laggy, specially after awhile of playing in them. You might try playing in Sunset Valley as a test to see if that helps.
Every now and again, I create a new Sims 3 folder to help keep things running well.
Try resetting the Sims 3 Folder, aka 3 Factory Reset (How-To)
http://www.carls-sims-4-guide.com/forum/index.php?topic=20732.msg401814#msg401814Also, you could try looking into this mod:
Smooth Patch - by LazyDuchess
https://modthesims.info/d/658759- Helps with loading the game, CAS and Build/Buy mode.
More tips can be found in the posts linked below.
====
Reference Links:
Game Won't Fully Load - Save slow loading
https://www.carls-sims-4-guide.com/forum/index.php?topic=20732.msg429056#msg429056Tips for General Crashing Issues
https://www.carls-sims-4-guide.com/forum/index.php?topic=20732.msg425173.html#msg425173