If your Mac starts running slow it could mean you have filled up your hard drive. Hard drive space means the space that is consumed by applications, photos, music, documents, etc.
Here is how you can check it.
Find the hard drive icon on your desktop and click on it once.
Right click (or hold down the control key while you click if you have a one button mouse) and a menu will open.
Select “Get Info”
You will find a line that gives you the capacity of your Mac and another that gives you the available space remaining on your machine.