Just adding my experience.

Departing SGN. Arrived at airport 7:30am with lots people already.
Was flying Cathay Pacific business. Asked the guard at priority immigration if I could enter and he said no. (Priority immigration had about 10 people in)
Regular immigration extended all the way to the entry point of the line but thankfully only took about 30 minutes.

Security line started immediately by the immigration desks. (Line had snaked around four times before the actual entry point for it)
This took 60minutes to clear. Lots of people trying to cut in which caused some shouting. No staff to manage. Also the room was uncomfortably warm.