Dovecot performance tuning
Disk I/O optimization
Usually heavily loaded IMAP and POP3 servers don't use much CPU, but they use all the disk I/O they can get. So reducing disk I/O is probably the most useful optimization you can do.
See MailLocation/LocalDisk for generic disk I/O optimizations.
See MailLocation/Mbox for mbox-specific optimizations.
See MailLocation/Maildir for Maildir-specific optimizations.
See dbox for Dovecot's own high-performance mailbox format. It usually gives much better performance than mbox/Maildir.
See full text search indexes for optimizing IMAP SEARCH command.
See POP3Server for POP3 optimizations, especially pop3_no_flag_updates=yes
CPU usage optimization
See LoginProcess for optimizing CPU usage caused by logins
See auth_cache_size setting for caching passdb and userdb lookups
Services having client_limit>1 and process_limit>1, set process_min_avail to the number of CPU cores
To reduce forks by reusing existing processes for new requests increase service { service_count } from 1 to higher (e.g. 100) for imap and pop3 services. It's better not to set it too high or unlimited (0), because different users use different amounts of memory, and it's wasteful when a lot of processes end up having a lot of "free" memory.
Memory usage optimization
There aren't many settings which affect Dovecot's memory usage. About the only such settings are auth_cache_size and LoginProcess settings. In general Dovecot uses as much memory as it needs, which is usually quite little.
Note that these settings do not directly affect the memory usage:
service { vsz_limit } : These are simply safe guards against potential memory leaks. If the process's virtual size reaches the limit, the process is killed by the kernel.
service { process_limit, client_limit } : These are mostly to avoid DoS attacks using up all your memory.