These are the things I look at when I'm having issues, maybe something in this list might be of help.
Lag can be caused by routing errors - when things have been placed by using the move objects cheats, or simply there is just way too much stuff on the lot. I recalled once that I downloaded an apartment complex from the gallery, and the lag to this lot was horrible. It was because the creator just had way too much stuff on this lot.
There are some files you can safely delete that might help with game issues, including this problem. Here is a link that goes into which files you can delete.
http://www.carls-sims-4-guide.com/help/cache.php
If you have a mod that is either outdated or is conflicting with another mod, this can causes issues as well.
