7.1.1.4   If Apple Mail Is Running Slowly

1. In Mail’s Preferences window, click on the Accounts tab, then your account, then the Advanced tab, and uncheck Automatically detect and maintain account settings.
2. If Mail is launching slowly and you are using macOS 10.14 or later, go to the General tab of Mail’s Preferences and click on Manage Plug-ins…. Uncheck SpamSieve.mailbundle and restart Mail. Then go back to Manage Plug-ins… and check SpamSieve.mailbundle. Now restart Mail again.
3. Change the AppleMailPlugInSetIsJunk option in SpamSieve’s esoteric preferences to not mark messages as junk.
4. If training is slow, see the Faster Apple Mail Training section.
5. If you are using the Apple Mail - Remote Training, Apple Mail - SaneBox, or Apple Mail - Server Junk Mailbox script, try unchecking the Mail rule that runs the script and see if that helps. You can also run the script in Script Editor to see if it is taking a long time to complete.
6. If none of these suggestions helped, please see the Sending in a “Sample” Report section.