We did find that the single rider for Rock N Roller coaster still took longer than expected because everyone eventually gets dumped into the loading area and back with everyone else waiting. Still when they call out for single riders as you get closer to the front it does help to by pass the last dozen or so people in line.
This is not accurate. Every time I have used single rider on RNRC, there is a separate entrance for the pre-show (in back on the balcony), and there is a separate line in the loading area. It is difficult to tell how long you're going to wait as a single rider, because from outside you can't see how many people are waiting. The line does tend to move slow. How long you wait depends on the time of day, crowd level, and how good the CMs are in loading the train. I've sometimes seen CMs send odd-numbered parties to the front, and even-numbered parties to the back. That really cuts down on the number of single riders needed.
Test Track single rider line moves quicker due to the nature of seating 3 across, lots of single riders are needed. Everest is the best single rider line in my experience. It's hidden around the corner, so some people probably don't even know about it. And you can see from outside exactly how many people are in line. There's also more rows on the trains than RNRC, which helps.
Basically, single rider lines will be shorter when the standby lines are shorter. The longer the standby line gets, the more likely a family will agree to split up to save some time. For example, is the standby line is 20-30 minutes, groups aren't likely to use the single rider line (but note at rope drop it's possible that single rider line might not be open right away, in particular the EE single rider line). But once those standby lines get up to 60 minutes, you'll see more groups in the single rider lines, making those lines 30-45 minutes (for TT and RNRC). I've used the EE single rider line many times and never waited more than 20 minutes.
As for other tips, for rides like Tower of Terror, Space Mountain, Soarin', TSMM, etc, pay attention to the CMs when you get close to the loading platform. As a single you can sometimes get pulled out of line early to fill in a car or row. And if you see an empty seat, let the CMs know you're a single and willing to take it.