I learned most of this when Anx was more active here and shared her wealth of knowledge on how to keep our precious babies running.
A clean Mac is a happy and fast Mac. One thing most of us hate to do is turn the whole machine off. However, just as we need sleep to resort our minds and thoughts, a Mac needs to be shut down so the various caches can be emptied of temporary files. Now if you are on this website you KNOW you are on the net and in your CS/PSE a lot. Every click can result in a stored temporary file. I try, note try, to close my browser daily and also to clear the cache as in Clear Recent History. Yes, you will have to sign in again to all those password sites but it's worth it for speed.
Totally turning you Mac off and not just shutting LR and/or CS/PSE is what you need to do when you start seeing your files get blurry or any of those programs gets slow or when beachballs or spinning blue balls appear more than you'd like.
Anke said she didn't know what Disc Utility was and it is an important tool for all Mac users.
Disc Utility is under Finder, Applications, Utilities, Disc Utilities.
Select your HD and then First Aid.
Repair Disc may not be available, so select Verify Disc. Hopefully, you will find that all is ok. If not then, you will have instructions to follow.
Do the same with Permissions.
If there ends up being something you can't repair here on your own, then a trip to the Apple store may be necessary or if you're still under warranty, a call to Apple.
NOTE !!! Any time you have to got to the Apple Store or call Apple, the first thing they will ask is if you have a current backup. If your Mac is working just fine, do a backup today. With all computers, it's not If, it's WHEN. Having a dedicated ehd for backup that is done often is really, really important.
Other easy but not so much fun things to do is keep your Mac's desktop as clean as possible. If you need a folder on the desktop, keep it elsewhere and make an alias that goes on your desktop. ( I have a dh who has a newer, faster, more memory MacBookPro, and he is always complaining about slowness. However, his MBP desktop is as cluttered as his physical desktops. 'nuff said. ) Get rid of files/folders you don't need. Yes, this takes time but it will free up memory every time you do it.