My impression was that it was more efficient, as it uses a constant connection. That means that a mail check is just a couple of packets on an existing connection, instead of a full connection setup/takedown sequence.My main objection is that it (actually IMAP in general, not just the UW imapd)) uses much more processor time per user than qpopper (or POP3 in general). That may be an issue on a heavily-loaded mail server.
The speed of processing mailboxes depends on the storage type used. mbox is notoriously inefficient. maildir works pretty well if the underlying filesystem handles large numbers of files in a directory well, but requires more per-message overhead (ie. an inode and the "slack" at the end of the file storage block). The UW-IMAP server can optionally use an alternate many-messages-per-file storage format that allows fast random access, and supplies matching utilities to store new mail in these formats, for use as an MDA.
Other IMAP servers can use custom DB-like formats to hold messages.
