In "classical" or "cartoon" animation, you create the effect of scrolling by moving the peg bar that holds the background layer(s) in increments of millimetres. To animate the slowest scrolling, you move the bg one millimetre per frame(either 24 or 30 frames per second). To create the illusion of faster scrolling, you just move bg in larger and larger increments.
With film/animation cameras/software, you "shoot" every single frame to fill up every frame of film/video, but really, you're just shooting identically frames all the time. In a video game, it would be the equivalent of rendering fewer frames.
So in a 2D video game, aside from how collision is handled, theoretically speed should be is easier to produce visually than slowmotion, since it requires fewer frames per second. I assume that most games usually use built-in in scrolling routines that don't offer the flexibility of custom scrolling rates, which could contradict this.
If Sonic wasn't stuck in a consistant framerate, it could scroll much faster. But the faster a game moves the more gameplay is lost, aside from human response time, fewer frames per second of visuals equals fewer frames per second of gameplay/collision/control. Which is why AM2 was reluctant to allow online play for so long with the Virtua Fighter series and how they explain that it has tighter gameplay than any other series(the gameplay is actually calculated frame by frame).