If you are approaching maximum capacity, click the Manage button to free up some space. You can check out how much free space remains on your MacBook's drive by clicking the Apple logo in the top left, select About This Mac and then click the Storage tab. The more crowded its SSD becomes, the slower it will run. Sometimes, all your MacBook needs is a data cleanup. With your Mac's drive freshly backed up, you may proceed.
Grab an external drive and run Time Machine. Before we begin, allow me a word of caution: back up your data before diving in.
