Do you have a ton of files, like documents or photos, saved directly to your hard drive? Slow performance may mean you're about to reach the storage limit on your Mac.
Solution: Check your hard drive space by clicking the the Apple icon in the top-left corner and then selecting "About this Mac." Next, toggle over to the "Storage" section and wait for it to calculate how much space you're using. If it's almost full, click the "Manage" button in the top-right corner.
This will open a new window with many tools for clearing out space on your hard drive, as well as readings on how much space every application, document, and more is taking up.
You should also consider using an external hard drive to clear out some space. Plus, it ensures that your stuff is protected even if your computer glitches or dies.