when someone says the pace is 18-20mph that doesnt mean you'lll always go that speed lol. That's the average speed and its more as an indicator of how fast the group will be in general.

My group of friends used to advertise the ride as 18-22mph, but we rode balls out on the flats(26+), climbed hard at 14+ mph and deescended like daredevils at 30+ mph. The average speed of the ride would be around 19-20mph usually...

It's called stop signs, waiting for people after climbs, and sometimes coasting.