[Exim] 1000 bcc email slow - why?

I'm sure it's in the manual somewhere but here's my situation and I
can't find the solution.

When I send a message to a large number of people the message will sit
there - apparently doing nothing - for a long time. Generally 30 minutes
for 250 names. Then - it wakes up and delivers these messages very fast.

I don't know what it's doing furing the 30 minute delay but I want to
turn off whatever it's doing and have it just send the messages. If it's
optimizing or verifying something then I want it to not optimize or
verify.

What do I need to do to speed this up? Sendmail doesn't have this delay.