I can only speak for EMA but the check-in/boarding procedures for easyJet involve having one PSA assisting pax with the self-service-check-in machines, and then one PSA who works on the bag-drop belt from just before 2 hours before STD. 30 minutes before departure, the two go through to the gate to board passengers, usually meaning the gate is set up 20/25 minutes before departure time.
The passengers who have made the effort to get to the airport early should be rewarded by having a low boarding pass number to get onboard first. This is the whole point of the open seating policy. Passengers who decide to use online-check-in, so that they can get to the airport as late as possible bypassing the airport hassle, will board last, but will not have to get to the airport early. So take your pick early & first aboard, or late & last. It's fair & will be easier to handle. I'm dreading to see the Ryanair terms & conditions for OLCI.