C<QUERY_WORKERS> defaults to the number of CPUs available, but 4 per
remote (HTTP/HTTPS) host.
-C<WRITE_WORKERS> defaults to the number of CPUs available for Maildir,
-IMAP/IMAPS, and mbox* destinations.
+C<WRITE_WORKERS> defaults to 75% of the number of CPUs available for
+Maildir and mbox* destinations, but 4 per IMAP/IMAPS host.
Omitting C<QUERY_WORKERS> but leaving the comma (C<,>) allows
one to only set C<WRITE_WORKERS>