As a proud owner of one of the slowest Macs that are able to run Mountain Lion (I have an early 2009 model, 2 GB Ram, 2.24 GHz (?) Intel Core 2 Duo and a NVidia 9400M), I have to say that, in contradiction to what I expected, everything is faster, especially when switching desktops. Switching desktops would sometimes lead to some frame drops in Lion, but everything is pretty smooth in ML.
This also applies to other system animations, including the lock icon animation in system preferences, which I have had problems with since Snow Leopard. This might also apply to games, but I haven't really tested that yet. As said by almost everyone, Safari is notably faster to, no scrolling problems whatsoever.
And last but not least is XCode, which builds faster than I remember and doesn't really seem to slow my Mac down anymore.