Grady, it seems like they get sent out in batches during peak times. Late at night and other off times, they pop right in immediately (a minute or two after the post hits, like you mentioned).
That's just how it is for me atleast. Sometimes I log into email and think that no one has hardly posted while I was away, but then there are 20-30 new posts on threads I posted on already.
It's just nice that the server doesnt bog down and keep us anymore IMHO.