It could be because of the amount of mods you have. Are they simply cc (e.g. clothes, hair, some build items like furniture, etc.) or are they actual mods like script mods and UI cheats (things that change the programming of the Sims...e.g. MC command centre)? If it's cc, then you might just have too much and should delete some, or if the mods are the problem, then you could either have too many or some that are broken/not up to date. The best way to test if your cc or mods are the problem is before startup, remove the folder (e.g. cc) from the game, then launch the sims and see if it's still slow; then the same thing with your mods folder. Let's say you're game is running faster without cc, then I would recommend downloading the Sims 4 Mod Manager (it's a separate application so it won't slow down your game down). The Mod Manager will allow you to see all the mods and cc you've downloaded, let you know if any cc needs to be updated, show you thumbnails, and you can even delete content you don't want in the application and it will transfer that information directly to the sims. Just make sure that the sims isn't running in the background or it won't work. This is the best way I know to clear up extra space in your game :)
