IME, they aren't always like this, but frequently very bad, and unfortunately quite unpredictable. We arrived on a very early flight a couple months ago and walked up directly to the counters with no wait at all, but this was purely a matter of luck. Looking at the monitors at baggage claim, five widebodies landed within minutes of our flight, and I'm sure the people on the last plane waited for a while. One antidote is to fly into smaller airports -- in the case of Vietnam, we had great success with PQC, although a friend just experienced a 20 minute wait there. DAD might still be ok in off-season, but is very busy nowadays in summer. When departing SGN/HAN, VN Business passengers can use the APEC/Diplomat lane, but this isn't posted clearly (or at least not easy to figure out in an insanely crowded immigration hall), so you have to know where to go. If you arrive at a peak time and aren't in VN Business, it's probably not a bad idea to use an expediter, although the cost I saw quoted in the PH Saigon thread was rather more than $25.

Few other things take the joy out of travel like arriving to an hour+ immigration wait. Over the last 20 years, I've had these lengthy waits frequently in Bali, Malaysia, and Macau (all of which have improved significantly), and these days Vietnam and Japan are my biggest problems. At least in Vietnam you can solve the problem with money (by flying VN Business or paying an expediter), but in Japan, the only option if you don't want to roll the dice on wasting two hours at HND/NRT/KIX is to fly into secondary airports like KOJ or SDJ.