How do I tell qmail to do more deliveries at once?

The default number of concurrent outboune email connections is 20, you may want to increase this number. To do this you need to put your chosen number into the following file:

/var/qmail/control/concurrencyremote

and restart qmail. Make sure you set the "descriptors" or "iles" resource limit to at least double the concurrency plus 5, and the "maxproc" resource limit (if your system has one) to at least the concurrency plus 4. Otherwise qmail will unnecessarily defer deliveries whenever a big burst of mail shows up.

Note that qmail also imposes a compile-time concurrency limit, 120 by default. You can change this in the conf-spawn file and then recompile qmail.