specifically from a filesystem IO perspective:
- new mail delivery
not much difference.
- searching a mailbox folder
if you do linear search then yes
- mass deletion of emails from one mailbox folder
3 is not true.
maildir is more IO efficient when reading and deleting individual emails.
and making backups. deleting from maildir means just delete a file, not shuffle data.
Everyone needs to make backups while it is unlinkely that anyone like every backup to be effectively full backup.
mbox puts the load on the mail server application and on memory.
and on I/O too - often quite a bit
maildir puts the load on the IO subsysetem. Which is precisely why Timo created the mdbox mail storage format, attempting to get the best of both worlds. And this is great idea and actually works :)
mbox may make sense for archive storage. you create archive folder once and never modify anything