I already have quite a large mailbox and I started running into timeout issues when searching in Roundcube mail. AI gave me some instructions for installing the dovecot-fts-xapian plugin, but I kept encountering various errors:

Warning: FTS Xapian: missing configuration - Using default values
Warning: FTS Xapian: Memory too low (text) 'std::bad_alloc'

The problem was incomplete configuration, but the AI didn’t know what to do. I found the correct instructions on GitHub.

(...)
protocols = imap pop3 sieve lmtp

mail_plugins = (...) fts fts_xapian

plugin {
    fts = xapian
    fts_xapian = verbose=0

    fts_autoindex = yes
    fts_enforced = yes

    (...)
}

service indexer-worker {
    # Increase vsz_limit to 2GB or above.
    # Or 0 if you have rather large memory usable on your server, which is preferred for performance)
    vsz_limit = 2G
    # This one must be 0
    process_limit = 0
}
(...)