SSD drives are very fast but expensive. So I have a crude idea that I'd like to see. May not be practical but would like to get some thoughts on it.
Some new SSDs use SATA 3 (6gb/sec) with 355/mb/sec read speeds and 215MB/sec write. Put these in raid 0 and it screams! Can you imagine how fast that would be?
What would be nice is if new email were on faster drives with old email being migrated to larger mechanical cheaper storage. Perhaps messages over a month old? From dovecot's perspective it would sort of all look the same but maybe one a week a script would run migrating older messages to slower media.
It would also make backups easier. The faster newer messages could be archived hourly. On the weekend after the archiving script was run the older message could be backed up once a week.
I'm not sure what it would take to make dovecot seamlessly access email from two different locations or if this is practical. Just wanted to throw the idea out there to see if something sticks.