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After installing the public beta of macOS Mojave, the Mail app became slow/unresponsive.
I have already restarted the system. It did not help.
What can I do?
After I chose "Mailbox" - "Rebuild", the problem was immediately solved.
Check Mail -> Preferences -> Rules. See thousands of duplicate rules?
If so, select all duplicates (shift-click), then hit Remove button.
Patience - takes awhile to remove them.
Quit Mail app, then reopen.