Why is a Mac so slow?

There are several reasons that could lead to a Mac running slowly, such as a full hard drive or an outdated OS X operating system. Other reasons include an overloaded program startup queue, too many processes running in the background or an outdated hardware setup.

If a Mac computer starts up fine but begins slowing down while in use, some potential reasons for this include insufficient RAM or having too many applications open at the same time. A desktop cluttered with icons and files also places a strain on the system, as each icon needs to be rendered over if any of them are moved or interacted with.