If your mac is running slow, you can view what is running on your Mac to see if it is slowing your computer down. Go to "Finder", then select "Utilities" under the "Go" menu and select "Activity Monitor". If that does not work, then the fact your system is running out of storage could cause this issue. To figure out how much storage your system has, go to the Apple menu and click "About This Mac". Then click the "Storage" tab to see how much free space is left on your system. If this does not work, then reset "NVRAM/PRAM" on your system. To do this, shutdown your Mac completely and then turn you Mac back on and them immediately hold down the "Option + "Command" + "P" and "R" keys. Do not release these keys until you hear the startup sound twice or see the Apple logo is gone for the second time.