1. Your computer’s startup disk may not have enough free disk space. To make disk space available, you can move files to another disk or an external storage device, then delete files you no longer need on the startup disk. macOS can also help you optimise storage by storing files in the cloud and helping you identify files you no longer need.

2. An app you’re using may require more memory than your Mac has easily available. To see how much memory your Mac has, choose Apple menu > About This Mac, then check the amount of memory listed in the Overview pane. See the documentation that came with the app to find out its system requirements.

To find out how to increase your computer’s memory, choose Apple menu > About This Mac, click Memory, then click Memory Upgrade Instructions in the lower-right corner. (Not available on all computers.)

3. Quit any app that isn’t compatible with your Mac. For example, an app may require a different processor or graphics card. See the documentation that came with the app for information about its system requirements.

To see what processor your computer has, choose Apple menu > About This Mac, then click Overview.

To see what graphics card your computer has, choose Apple menu > About This Mac, then click Displays. The name of the graphics card in your computer is displayed in the middle of the window.

4. Quit any apps you’re not using.

5. If you have problems with your disk, use Disk Utility to verify and try to fix any issues.

6. Use Activity Monitor to see if a particular app or activity is making a large demand on your Mac.\

SOURCE: "Why is My MacBook Pro So Slow"