**While not always the case** I think airports have an impact on boarding. When I say airports the specific gate area being used for the flight.
Few have enough space for adequate lines to form.
I just got back from two weeks in Europe where I got to experience DL service as well as Vueling and Alitalia.
1- Delta's boarding process at MAD was good, mainly because they are in T1 and had a HUGE boarding area for the two lines to form. While I have no issues with DM boarding with Premium, on D1 flights I think D1 should board first. With (40) seats that's a large group to board first, not counting DM's who are in back.
2- My Vueling flight BCN-NCE was efficient. They board rows 15+ and then 1-14. They did do pre-boards and people who paid for early boarding (which was like 3 people) This flight was out of the gate on time.. to the minute!
3- Alitalia was a disaster. Part of this is because of the fire at FCO and lack of gates. A HUGE line formed 30 minutes before boarding and they did call SkyPriority, but there was no second lane. We were all getting on buses to go to remote stands, so priority meant nothing. Our bus first stopped at the wrong plane before finally figuring out where it needed to go. This flight left 30 minutes late because of the boarding process.
I will say on all of my Delta flights, including domestic connections, the gate agents enforced the premium boarding zone. I saw several people turned away. This is what DL needs to do across the board and people will learn.