Dovecot v1.0's deliver updates the main index file while message is being saved. This is useful with mbox format, especially if mbox_very_dirty_syncs=no. With Maildir the benefits of this are pretty small.
Dovecot v1.1+ deliver updates also cache file, which can be very useful with all mailbox formats. It means that when IMAP client wants to fetch the message's metadata (e.g. some header fields) they're already found from the cache file and Dovecot doesn't have to open and parse the message file. There are some tradeoffs though:
- LDA indexing wastes disk I/O because it has to open and update index files
- LDA indexing saves disk I/O because it already has the message body in memory, so it doesn't need to read it from disk.
- IMAP indexing wastes disk I/O because it has to open and read message files
- IMAP indexing may save disk I/O because IMAP process always has index files opened, and many IMAP clients are configured to download all new message bodies anyway, so the second time message bodies are read they're already in memory
So it depends on IMAP client if it's faster to use LDA or IMAP time indexing. In any case the user experience is typically faster with LDA indexing, because the message list metadata can be returned faster when it's pre-indexed.
See IndexFiles for more information about what the index files contain.
Non-indexed mail delivery
Ignoring the benefits of cache file updates, the only thing left is the main index updates. As mentioned above, with Maildir format these benefits are very small. This also means that it's perfectly fine to use a non-Dovecot MDA to deliver mails that doesn't update indexes. Dovecot can efficiently see and index such new mails without doing anything expensive like "rebuilding indexes".