@PetitDavid4487
FPS will always drop when you do certain things. Like go to Build mode from Live mode and back to Live, go to CAS from Live mode and back to Live, or take photos using the in-game cameras or tripod. FPS will usually come right back up in a few seconds after going back to Live mode but not always. The easiest and quickest thing you can do is to hit the Escape key to bring it back up to where it ought to be.
If the low FPS persists, then another thing you can do is shut down, delete the localthumbcache file and restart. Another thing you can do is change your graphics settings to a lower setting.
FPS will vary on each lot as well. Larger lots with lots of objects and sims will usually run at lower FPS than smaller lots with fewer sims. So, another thing you can try if the problem is only happening on certain lots is to declutter the lot and reduce the number of objects. Or play smaller households on those lots.
Hope this helps.