What does the term "Speed" mean in a speaker?

I think our perception of speed has more to do with how fast a speaker driver stops making sound after the signal stops than how fast it reacts to a signal. That's why larger drivers often sound slower than smaller drivers. It's not too difficult to get a driver to respond quickly when its voice coil is energized. It's a lot harder to get it to stop vibrating once the signal goes away. 

Servo controlled drivers can overcome some of these limitations because they can use power to stop the driver quickly when the input signal stops. 

Another way of talking about "speed" is "lack of stored energy."

That is, the waterfall plot is very clean and very short, without ringing.

Not to brag, but...

My subwoofers are so fast, they can do a 20-cycle sine wave in a MERE fifty milliseconds.

Not only that, I’ll go out on a limb and claim NOBODY makes a subwoofer that can complete a 20-cycle sine wave in less time than mine.

;^)

Seriously, I agree with those who have noted in various ways that what happens with the trailing edge of the note - how quickly and cleanly it decays - is (in general) more important than what happens at the leading edge of the note, at least when it comes to the subjective impression of "speed".

Duke

Another way of talking about "speed" is "lack of stored energy."

I supposed that has more to do with "driver break up" or suffering from "impedance anomaly".  Paper cones may have a clean water fall plot, but an aluminum cones may have "faster speed" but may not look as clean on the water fall plot.  

Of course having "faster speed" does not always mean "better".  There word "speed" in this thread seems to have a lot of different interpretations.  Different people seem to have different impression of the word.

Now I get. The view perpendicular to the revolution is opposite except in  the downward position.