Mail is taking forever to load messages. Why? addendum
I found this email thread when I asked the googlesphere about slow Mail files loading. I tried the EtreCheck tool and it reported some unsigned Oracle java code files - and since I recently attempted to update java, I deleted those and then deleted all of Oracle java (via terminal: sudo rm) to see if might be the problem. After a restart of the system and a couple times restarting Mail, the lag time in loading has (apparently) ceased.
as we say YMMV.
MacBook Air 13″, macOS 12.7