Danang was running late by the time we got there, and they kept letting the folks that needed to board ASAP through the line. Ultimately that meant that I was standing at the front of the queue for about 25 minutes (it might have been longer). The line didn't end there-- there were only 2 X-ray machines for the whole airport. Ugh.
Ho Chi Minh's line wasn't too bad, however, for Vietnam Airlines you queue in front of the booth instead of having one line that feeds to multiple booths. After much waiting (because they also needed to verify my visa to Myanmar), I finally got my boarding pass, and then I assumed it would be smooth sailing. Alas, the immigration queue was not much better. Again the same system-- each line only fed into the booth it was in front of. Worse yet, families in a rush tended to cut the queue (they didn't realize how long it actually was) and so a few of us had to keep telling folks to go behind (a "perk" to being close to that area for a long time). More waiting to get through security (only 2 X-ray machines for each half of the terminal-- so 4 X-ray machines total). Once through, we rushed to find some breakfast and our gate.
In short, these airports seem fairly new, and no one has really engineered the system to be efficient. They've definitely got enough staff and enough space.... Just be sure you're early (or really maybe being late is better if they actually bring you to the front of the queue). Utter chaos.