Just experience Hagrid;s today. The sign said 50 Minutes but we got off the ride at 33 Minutes. We did notice that a group that got in the single rider line right after we got in the regular queue and they got on the ride right after we did. So really no time saver this time around for the single riders.
I don't know that there is any true way to know if the single rider will be shorter, the same, or longer.
A few years ago on a Christmas trip, Our family for in line for Hagrid's and were stopped just steps inside the indoor part of the queue. The ladies behind us left after a few minutes. I don't remember how long we waited, but it was probably an hour or so.
As we got to the boarding area, since we were 5 people, one of our party got paired with a single - it was one of the ladies that had been behind us in the regular line. We laughed about getting there at the same time when she said, "Oh... no... this is my second ride"
On some rides, it's very hard to know if the single rider will be faster or not.
On our last two trips, on each ride of Velocicoaster, I watch the people getting in single rider at the same time we get in the standby line. I mentally mark 2 or 3 different people - that do not appear to be together - in that line so I can be reasonably sure they aren't going through for a second time when we see them again at load.
Almost every time, we're within 2 or 3 trains of each other. Sometimes standby is faster, sometimes slower. Once, however, we were boarding and the single riders I keyed on were 20+ people away from boarding.
I'd bet, at some other point during that same day, the opposite happened.