The official RER schedules are found here:
http://www.ratp.fr/horaires/fr/ratp/rer

Typically the Gare du Nord to CDG non stops take 30 minutes to Terminal 2. Trains making all of the stops en route typically take 35 minutes.

While RER B trains do not overtake one another, the trains which depart ahead of any express trains are those going to Mitry-Claye and take a different route at Aulnay sous Bois, thus clearing the way for the express trains. I have never heard that express trains are slower in speed than are other trains.