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.
CPU usage optimization
See LoginProcess for optimizing CPU usage caused by logins
See auth_cache_size setting for caching passdb and userdb lookups
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:
login/mail/auth_process_size: 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.
login_max_processes_count, max_mail_processes and auth_worker_max_count: These are mostly to avoid DoS attacks using up all your memory.