Screamin's single line can be incredibly short. I've had that experience twice. Hubby got caught in an odd situation with a CM in a bad mood, and he had a longer wait. Not anywhere near as long as he would have waited first for the FP window to open and then to wait in the FP line, though.
That Single line has you get a pass from the CM at the start of the line, then you leave that area, go up the hill, and into the accessible, gated, entrance. That puts you on the opposite side of the loading platform. As the loading CMs find empty spaces where the 3rd of 3, 5th of 5, etc, or Singles who didn't know about the single line, are leaving an open spot, they call over the next Single Rider. For me it was very fast each time.
With hubby's situation, there were too few empty spots, and the Single Rider line was getting long. I guess they decided to move those in line, so there was no backup, so they had the Single line go into the elevator, go over the track, then down the elevator, so they were in a holding pen area on the normal side of the loading platform. That would have worked beautifully if the CM hadn't *ignored* them. They actually started calling people from the newly formed proper single rider line to fill in spaces, while those in hubby's group were kept waiting and waiting. Finally someone got the CM's attention and told him what was happening, and the CM just let them all find spots and go. But I think that was an aberration. Still, it's worth mentioning.
Soarin' is weird, b/c you will go in and just walk past *everyone*. Everyone will glare at you, wondering why you're breaking the rules. And then you will stop when you are told to stop, and then they put everyone into the queues at each "gate", and everyone who glared at you will look at you with "see, shouldn't have broken the rules" in their eyes.

I like keeping a Buddha-like smile on my face. One lady asked me what I was doing once, and she was really intrigued by it (they were Dream Fast Pass winners and they waited longer than I did for the ride). Then as they find empty spots, they call the Single riders up.
I've done Splash too, and it was fine. For the single rider issue alone, I'm glad they have those dividers between each person. It was weird enough sitting behind some guy I didn't know, let alone if he'd been right up against me!
I've never been there at a time when GRR needs the Single line open (I've never even needed a FP there), so I just get in line and they plop you in a rat with strangers and it's very fun.