The "immediate" emails are picked up by the Timer service and I think it
runs every five minutes or so. If you would like to increate that
interval you can do it with STSADM. Use a command like the following:

stsadm -o setproperty -pn job-immediate-alerts -url http://servername
-pv "every 1 minutes between 0 and 59"

That will reduce it to every 1 minute. The only downside to this is
more database access, but that is probably not a concern.