The amount of time between forking a process to run through the queue is defined by the -q flag. If you run with delivery mode set to i or b this can be relatively large, since it will only be relevant when a host that was down comes back up. If you run in q mode it should be relatively short, since it defines the maximum amount of time that a message may sit in the queue. (See also the MinQueueAge option, as described in ``O: set option''.)
RFC1123 section 5.3.1.1 says that this value should be at least 30 minutes (although that probably does not make sense if you use queue-only mode).