Mail messages take half a minute to open.
Why does it take so long for my messages to open in mail?
iMac 21.5″, macOS 12.3
Question marked as
Safe Boot from the HD, (holding Shift key down at startup), does the problem occur in Safe Mode?
Safe mode attempts to repair Disks & clears lots of caches, so if Safe Mode works try again in regular boot.
If you have no POP accounts & they’re all IMAP accounts then in Mail’s Mailbox menu choose Rebuild.