I think there is a main factor (or two) in what makes what should be a simple effect look bad. While speeding up the film 2x, the vehicles appear to move twice as fast, which is definitely feasible and something most of us humans have a reference for (I’m sure all of us have seen fast-moving objects). What ultimately ruins the effect is that by increasing the speed 2x, the force of gravity is also multiplied 2x, and our brain simply can’t accept what looks like a vehicle going 90mph getting pulled down by gravity like it’s going slower/weighs twice as much. This creates an unreal mixture that our eyes are unable to perceive as real, as we have seen otherwise. It doesn’t help that the camera pans unusually fast/smooth or people doing things much quicker than usual.
