mutt 20 times faster than mail.mailutils?

Say, I notice, for the time it takes to get the screen all set up for
the user to start entering commands,
$ set imaps://xxx%40yyy.zzz:[email protected]/INBOX
$ mutt -f $@ # is 20 times faster than
$ mail -f $@ # for a mailbox with 50 messages.
It seems mail.mailutils is doing some large loop for each message, while
mutt does it once for all of them.
Connecting is about the same speed, but there is a small delay for
mail.mailutils between printing each line of headers.

OK I just tried it with a 500 message mailbox.
Speeds unchanged. Still that delay when printing each line.