I run a small site and in my experience spam bots tend to be very quick in posting their spam messages.

Being able to run a short delay in the ability to post would be especially powerful against human spammers as they seem to move between websites very quickly. My guess is that in a boiling room the aim is to maximize the number of spam posts per working hour so even a 3 minute delay would take care of both the bots and some human spammers unless you run a major website.