[imp] IMP takes too much time to open Inbox.

[Scot Wilcoxon]

Mail servers, and IMAP servers, can work in several ways.  Your problem
where the list of messages is slowed by having large messages sounds
like your servers are using a single file for all that person's mail, so
the entire file has to be examined to extract the header information.

Some mail servers can store messages in other formats, or store messages
in database servers.

POP servers tend to have few abilities for mail storage formats,
although a mail server may include a compatible POP/IMAP server.

Some IMAP servers can use more efficient formats, particularly for the
Inbox.  Look at the documentation for your IMAP server -- looking at
one, I see /usr/share/doc/*imap* contains documentation for about a
half-dozen Inbox formats.

Look at the mail server and IMAP server that you're using and see what
options you already have.