Macs can perform slowly, too. If your Mac seems to be running slower than it should, the variety of tips here should help you identify and fix the problem. As with a Windows PC, there are many reasons a Mac might be slow.
Stay away from programs that promise to “optimize” your Mac and make it run faster. Some “cleaning” programs — like the Mac version of CCleaner — can remove temporary files and free up space, but they won’t make your Mac perform faster.
Find Resource-Hungry Processes
Use the Activity Monitor — sort of like the Task Manager on Windows — to view your running processes and find ones devouring too many resources. To launch it, press Command+Space to open Spotlight search, type Activity Monitor, and press Enter.
Click the “% CPU” heading to sort by CPU usage and see the running applications and processes using the most CPU. In some cases, there may be a single runaway application using 99% CPU that you’ll want to end. To force-quit a process, select it by clicking it and click the X button on the toolbar. Be sure you aren’t quitting a process that’s doing something important. You can always try to close a resource-hungry app in the normal way first.
If this isn’t working, click the “View” menu and select “All Processes” to see all the processes running on your Mac. You can also click over to the Memory section — a process using a large amount of memory could cause your Mac to slow down. Try the “Disk” section, too — a process using the disk heavily could also be causing your Mac to slow down.
Mac OS X likes to leave applications running in the dock. Even clicking the red “X” button on an application window won’t close it — it’ll remain running in the background. In most cases, this isn’t a big problem. However, if your Mac appears to be running slowly, you may wnt to close some of these applications.
Look for the applications marked with a dot on your dock, right-click or Ctrl-click their icons, and select “Quit.”
Prune Startup Programs
If your Mac is slow after you log in, it may have too many startup programs.
To manage startup programs, open the System Preferences window by clicking the Apple menu icon and selecting “System Preferences.” Click the “Users & groups” icon, select your current user account, and click “Login Items.” Uncheck any applications you don’t want starting when you log in.
If you ever want to make a program automatically start when you log into your Mac, drag-and-drop it into this list or click the “+” button at the bottom of the list and add it.
Read more: https://www.howtogeek.com/228180/10-quick-ways-to-speed-up-a-slow-mac/