This is something that's been kind of bothering me for a while. My first attempt to clear this up was by distinguishing "alternate path" from "shortcut," but that doesn't nearly cover it. However, I reached that conclusion before I had taken a good, hard look at fast zones—I said slow zones don't have alternate paths before I had stumbled upon the idea of path interaction (falling from a high path, falling to a low path, etc.). So, I might be tempted to say that path interaction (which is what I'll be saying until someone thinks of a better term for it) makes a zone fast, but that doesn't work, either. Path interaction is minimal in Scrap Brain act 1, but it's present. There are three segments of floating blocks near the center top of the stage that allow the player to fall from the top to the middle. The player never falls to the bottom except by choice, though. Still—more path interaction than what you see in Scrap Brain act 2. That's still kind of a problem, though. By my count, there are two places in Scrap Brain act 3 where the player can fall from the shortcut to the main path, and there's a fair bit of interaction between alternate paths on the far left side of the stage. At the end of the day, path interaction is never going to set the pace of a zone—it'll never be absolutely crucial to how fast or slow a zone is. How fast a player can go is not directly reliant upon how many multiple paths are present and how they interact. Path interaction is more present in fast zones and less present in slow zones, but there's no hard and fast rule—heck, path interaction itself is a pretty vague concept. I still think the fast - slow - fast - slow - fast - Scrap Brain zone order holds, but tying it to path interaction won't do. At least for now. This was just the kind of confidence breaker I was looking for, too. I've been holding back on finalizing these thoughts because they didn't feel right, but I just couldn't grasp how they didn't feel right. I hadn't noticed, but this is definitely true. In a sense, a ceiling gives them more real estate to work with. I'm not awesomely familiar with Sonic CD, so I'll take your word here. It's definitely an interesting concept, especially given the story of Sonic CD's development. I'll have to keep this in mind and give Sonic CD another playthrough.