@Lohla01 There's definitely a serious crashing issue currently in the game, and as far as I know, there's no fix yet. From what I understand, that wouldn't cause your save(s) to run slowly, just for otherwise normal saves to crash for seemingly no reason. This is the very, very long master thread on the issue:
https://answers.ea.com/t5/Technical-Issues-PC/CURRENT-ISSUE-Game-Crashes-after-November-2020-Patch/m-p/9719086#M193439
As you can see from the accepted solution (marked for visibility) on page one, EA is looking for saves from affected players, if you'd like to provide one.
If you're not convinced that this is your issue and would like to experiment a bit, you can try playing while your computer is offline. Sign into Origin and put it in offline mode, then disable wifi and/or unplug the ethernet cable before launching the game. That may not have any effect on your old save, but it would be interesting to know whether it helped in a new save.
For the old save, you could just put it aside for now and wait until the other issues have been fixed. Or you could try loading a backup to see whether it runs any better.
https://sims4.crinrict.com/eng/2014/09/save-games/#wiederherstellen
It's a good idea to use the manual method and make a copy of your backup saves, in case you need them later.