Too many custom content files can cause lag in Sims games, especially if you're the type of gamer who collects dozens and dozens of files. To prevent this lag, you can combine your custom items into a single package file.
You can find s4pe here: http://sims4group.github.io/. Download s4pe from that site and then use 7-zip to extract it into a single folder on your desktop. Once it's unzipped, you'll see that one of the contents says "s4pe.exe." Click on that to open the program.
Click on "File/New," and then "Resource/Import/From Package."
Find the package files on your computer that you want to combine. You can shift-click on all at once to bring them into s4pe all at the same time, or bring each in individually. It doesn't matter.
A new window in s4pe will pop up. It should be set on "Replace Duplicates." Click on "Import." Click on "No" when it asks if you want to auto-save.
You will get several lines of code that begin with "UNKN," "_IMG," or "CASP." Don't click on any of these. Click on "File/Save As." Name your new combined package and save it to your desktop.