When I was in DPS about a year ago there was a separate counter just for pre-issued visas which had a very short line (and VIP used this same counter) ... so at least then having the visa prior to arrival would have made a big difference. Perhaps the counters have changed since then (they seem to every few months). In any case my several trips to DPS have never encountered the huge lines, however I did note severe clustering in arrival time for some flights, esp. the LCC flights, so I'm sure the lines go from none to hundreds of people within a few minutes.