If your hard drive is getting full, it is recommended by some to keep at least 20GB of free space on your hard drive. The main reason for this is to be able to have enough room to do another upgrade of the operating system. Each new upgrade requires a minimum of about 6 GB of space just to download the file, and some additional free space is needed for the final installation. A full hard drive does NOT mean your Mac will run slow, but it could also be that the files on the drive are not optimized and/or contiguous.
John R. Carter, Sr.