Airmail App uses a feature called Advanced Graphics, to enhance the visual aspect of the app. However on older computers, this can slow the computer down.
To turn off this feature, please follow the following steps:
1. Open Airmail App
2. Open Airmail Preferences (⌘,)
3. Click on the Advanced Tab
4. Click on the select box to deselect Advanced Graphics
5. Close the window
6. Click on Airmail in the Menu Bar
7. Click on Quit Airmail (⌘Q)
8. Reopen airmail
Advanced Graphics has now been disabled.