All Macs begin to slow down with age — MacBook Pro, iMac. It doesn’t matter. I've listed some of the reasons below that may cause the Mac to slow down and found several ways to make your Mac faster. You can try.
1. Clean up your hard drive
2. Update your macOS.
3. Manage your Startup Items
4. End processes with Activity Monitor
5. Upgrade your hardware
6. Use the Mac cleanup software. Such as Mac Cleaner of iMyMac
7. Plow through your desktop items and Clean the desktop
8. Remove extensions to make your Mac run faster
9. Clean lots of outdated cache files
10. Finder uses too much memory and Adjust Finder preferences
11.Animations and rich graphics are extremely resource-hungry. So cut down on animations
12. Reset SMC to factory settings
13. Too much temporary files in memory. So restart your Mac regularly
14. Turn off the FileVault encryption
15.Over time, permission problems occur, causing your Mac OS X to lag, freeze and even crash. Then You can repair disk permissions.
16. Clean up Downloads and Trash folders
17. Clean up Preference Panes
I hope the above method can help you.