Fix File and Directory Problems
Although less common than many people imagine, it’s possible for things like corrupted fonts, caches, and preference files to result, somewhat indirectly, in a slow Mac. Similarly, disk errors of various sorts, such as directory damage, can result in a slower Mac (or cause other problems).
Fortunately, the majority of file and directory problems are easily solved—as long as you have the right tools and know how to use them. Because these sorts of problems can affect everything your Mac does, you should check for, and repair, file and directory issues before moving on to the more specific steps I cover in later chapters.
Run Disk Utility
Disk Utility, included with every Mac, can repair a variety of disk errors. ...