Noi Bai (HAN), Hanoi - 45 to 90 minutes at peak
Terminal 2 handles all international arrivals. The worst queues are early morning when European flights land together (around 5-8am) and the evening East Asia bank (7-10pm). Off-peak midday arrivals can be through in 20-30 minutes. There are few e-gates for foreign passports, so almost everyone queues at staffed counters.
Tan Son Nhat (SGN), Ho Chi Minh City - the longest queues in Vietnam
SGN has the country's worst immigration waits: 60-100 minutes is common in the evening and at weekends, and holidays can push it further. The international terminal is also the most crowded. If you arrive 6pm-11pm or on a weekend, budget real time for the queue - or use a priority lane.
Da Nang (DAD) - shorter but still spiky
Da Nang is smaller and usually faster: 20-40 minutes is normal. It still spikes during Tet, summer holidays and when several charter flights land together, especially Chinese and Korean tour groups.
What makes the queue faster
Fill in your arrival form on the plane if your airline hands it out. Have your e-visa printout or approval letter ready. Families with infants and diplomats can use priority lanes in some terminals. The reliable way to skip the queue entirely is a fast track meet & greet: an agent meets you at the gate and escorts you through the priority lane - typically 10-15 minutes gate to exit.
When fast track is worth the money
It pays off at SGN almost any evening, at HAN during peak banks, and everywhere during Tet and public holidays. It is also worth it with elderly travelers, small kids, or tight onward connections. On a quiet midday Da Nang arrival you can probably skip it.