I often heard people say the USO normally is the fastest of the 4 Slams, despite the higher bounce (and not counting last year's conditions), while the AO despite the lower bounce, are slower. I think the eventual speed of the surface also (or maybe even primarily) depends on the elasticity/spring of the material used. A different composition of the soil (and grass) at Wimby could cause different elasticity (however hard to imagine it comes close to a hard court).
...I would like to learn more about that.
I'm not a specialist at all but here's how I understand it.
yes there's a difference between two aspects : highness of the bounce and actual speed of the ball (speed in any direction, vertical, horizontal) which depends on how much the energy of the ball is taken by the ground during the bounce and also on how the ground reacts to the spin (because the spin requires a lot of energy in itself independtly from the speed of the ball, that is of the gravity centre of the ball).
However, when the bounce is lower, it makes it much more difficult for players to catch the ball in defense ... because the second bounce will be earlier.
And as far as the bounce is concerned, the main characteristic of grass which makes it better for attacking players and less good for defensive players is the fact that the bounce is low : the ball slips on the ground.
Another characteristic of grass in the past was that there were more bad bounces, which was also better for players who played serve-and-volley, McEnroe always insists on that aspect.
All of those aspects make that the "speed" of the surface also depends on the character of the bounce (vertical or horizontal direction, reaction to the spin), which is clearly something Wimbledon's groundman decided to take out of consideration.
And all of those aspects also explain why the word "speed of the surface" is often used in two different meanings.
But anyway the highness of the bounce is very important (for instance the WTF surface is quite "slow" as far as the speed of the ball is concerned but the bounce is low), and so are the balls.
And as far as grass is concerned, the highness of the bounce is at the core of the "topic" rather than what he calls "the speed of the ball", which means how much the energy is taken by the bounce. Compare how the ball slips on a "quick grass" and how high the bounce is now on the baseline in the second week of Wimbledon : it's 100% different for a Nadal !!
As for the Australian open comparing to the US Open, I'm not that sure of what you say since it has become plexicushion : it seems to me the bounce is not much different but clearly the US Open is quicker.