The compromise here is that it needs to be slow enough to be safe, but you also want to get all the drivers in one group so the recovery can start and have a big gap between passes each lap. The marshals can start the clean up earlier and the drivers are more likely to get more race laps.
The most common problem I've seen here is a driver going so slowly he doesn't catch up with the SC! Being the one behind that can be very frustrating. Drivers who'd have 'em
Not that this is a reason to not doing anything, or an argument that one method is better than another.