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?
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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. Faster?