I don't know exactly why, but experience suggests that passengers on Easybus or O'Leary airlines get to the gate and on to the 'plane promptly. I cannot remember the last time I sat on one of these planes waiting for passengers.
It may be that because they discourage hold luggage, they can therefore simply close the gate on time and leave without the passengers as there are no bags to remove.
Monarch, who allocate seats, in my experience, regularly sit around waiting for the last few passengers. Their flights are generally longer than the others, and so more passengers will be flying with hold luggage.