I happen to work on behalf of a certain orange outfit and their so called priority boarding system is by no means fast!
4 boarding groups result in 4 announcements. Not to mention having to turn people away that "didn't understand"
If you allocate seats, it can be done just as quickly. If you're boarding with front and rear steps, you advise which steps correspond to which rows in your announcement. If you're only using the L1 door then board the rear first, simple!
If you have a computer system that allocates seats, that costs money to buy and maintain. It costs hardware to run it on and then you have to print tickets. You have disputes at the check in desk of who wants to sit where and who booked what kind of seat on the web site. That takes time. Thus Money, Money, Money!
With the exception of Ryanair, (until the end of the year) all of the EU low-cost airlines are using a computerised Departure Control System, be it their native system, or that belonging to the handling agent. To create a flight that has Assigned seating requires nothing more than the touch of a button.
I always thought that the reason was that if you have your seat allocated then you hang around the airport buying that last bottle of whisky / having that final cup of coffee.
If you don't then you make sure you are at the gate nice and early.
Therefore allocated seats mean that you may have an aircraft sitting on the ground waiting for late passengers.
In my experience this is not the case. You will get late passengers whatever the outfit. 9 times out of 10 people who are late through their own stupidity couldn't give a to** about where they sit on the aircraft!