Easyjet do not always check boarding passes then divide passengers into priority/normal groups. They do this, for example, at AMS where the gates they usually use (H pier) are built with a holding pen past the boarding pass scanner.
However at Stansted and other airports, there is no segregated holding pen past the boarding pass scanner, only a passageway to the stairs or airbridge (depending on which is in use).
There is, in all Easyjet locations, a clearly marked set of two queues, one for priority and one for normal. When processing boarding cards, the gate agents start with the priority queue. Before boarding they actively marshal waiting passengers into the two queues, which prevents an impenetrable scrum at the scanner.
Meanwhile, in San Francisco recently boarding the BA286 (the A380, which does not board premium passengers directly from the lounge) there was exactly such an impenetrable scrum at gate A9 with no marshalling of queues.
It is not hard. Why can't BA's gate agents do it?