Enable maildir_very_dirty_syncs rather than just adding comment

data/conf/dovecot/dovecot.conf

@@ -25,9 +25,10 @@ mail_attachment_fs = crypt:set_prefix=mail_crypt_global:posix:
 mail_attachment_dir = /var/attachments
 mail_attachment_min_size = 128k
 # Significantly speeds up very large mailboxes, but is only safe to enable if
-# you do not manually modify the files in the `cur` directories in 
-# mailcowdockerized_vmail-vol-1. Disabled by default.
-#maildir_very_dirty_syncs=yes
+# you do not manually modify the files in the `cur` directories in
+# mailcowdockerized_vmail-vol-1.
+# https://mailcow.github.io/mailcow-dockerized-docs/u_e-dovecot-perf/
+maildir_very_dirty_syncs=yes
 
 # Dovecot 2.2
 #ssl_protocols = !SSLv3