IMO this change is not about saving time but making more money now that Ryanair's finances are being squeezed by strikes, EU261 etc The present system isn't broken and doesn't need fixing.
The priority/economy queue split is often about a 50/50, allowing the priority passengers on board first with all their baggage first that slows the process down. Dropping your economy bag onto a trolley is a very quick process and takes no additional time at all. Having done that you inevitably find yourself waiting to board for quite a few minutes while priority passengers load their bags into the overhead locker bins at the front of the aircraft. Having done that with the beast from the east howling across the ramp at MAN earlier this year still brings back memories of being absolutely frozen to the bone.
To speed the process up they should always use both front and rear doors, which they don't, and priority should use the front and economy should use the rear door.
The inevitable consequence of forcing more and more passengers to pay for priority boarding will result in unsustainable self defeat. I'll predict a year, max, before more changes and more charges.