RAM for mods and CC

@ClaraGreta  The overheating alone could account for the lag.  Modern computer components are very good at throttling their performance when their temperatures get near the danger zone, which is a good thing, but you'll see the effects of the throttling in-game.

Having a lot of custom content won't overheat a computer though.  Having high-poly items can demand more of the graphics card or chip, and possibly overheat it if cooling isn't sufficient.  The processor could overheat if its overall workload is too high, which could be due to Sims 4 plus other running processes.  It's not usually going to be cc-related, although having broken items might increase the load more than working cc would.

The amount of cc you have might increase RAM use enough to cause lag.  While each item has a negligible impact, and even a large collection such as yours still has a relatively small impact, it's possible your current computer is so close to its limit already that your cc collection is pushing it over the edge.

But this is all speculation.  I'd need to see the results of some hardware monitoring to know for sure, and on even a mid-range new desktop, none of the above should be a problem.