Very good question. One might expect that it would be that simple. Unfortunately it's not.
As it turns out, there is a limit to how much music can be sped up or slowed down (more so the latter), beyond which it becomes unbearable to listen to. This is to some extent a subjective matter, depending on the tastes of the listener and the quality of the music. But the more extreme the speed change, the more people will agree on the result.
There is a point at which nobody would enjoy any song slowed down, and let's just say for the sake of argument that that point is somewhere around -15%. Even if I'm off my as much as 5%, that's nowhere near the amount you'd need to slow down a Viennese to convert it to Waltz speed, since a Waltz is about half the tempo of Viennese.
Regards, Jonathan Atkinson www.ballroomdancers.com