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

mass and inertia matter, but you missed one parameter, driver surface area, which determines the amount of excursion for a particular level of volume, which goes a long way determining the linearity of the response.

mostly in the mid bass you find one driver, maybe two. or two crossed over. my speakers each have -4- 97db, 7ohm, 11" ceramic matrix woofers. covering 40hz--250hz. with all that surface area and a very stiff light ceramic membrane, the need just a tiny bit of excursion so they stay linear. and the amplifier is not stressed by the load with 97db efficiency.

so you get planar or horn type speed, but dynamic cone impact. images have weight and authority. tonality is maintained and not washed out.

i think speed in a speaker mostly refers to the mid and upper bass 50hz-250hz region which are the power frequencies where the music lives. vocals, drum kits, pianos, cellos, horns.....they all fall down or rise in this area.

leading edge precision and the presence or lack there of ease and refinement in these frequencies either impart flow and energy to the music and maintain the timing or muddle and restrict that flow and energy.

this is a speaker-amplifier-room issue, not just the speaker. and typically you see a crossover right here, amps struggle controlling the drivers here, and rooms have most of their worst bass nodes in this area. as you increase the dynamics and SPL’s this will be where things go to hell first as the combination of the speaker’s limitations, the amp and the room acoustics all rear their ugly heads.

but get the crossover out of this region, have sufficient driver surface to limit the need for much excursion, and appropriate amplifier for the speaker draw, solve the room issues, and you can get the ease and effortlessness and the speed and precision of the music will result in that speed that serves the music, the music breathes and soars.

Mike, unfortunately regular dynamic drivers are a poor impedance match to air. They have to work much harder to get the job done. ESLs and Horns do not have this problem to near the degree.
ESLs and horns are generally described as being very detailed. They also have better transient response, association or causation. I would say the later. Yes, a speaker with a lighter moving system could have better transient response assuming the motor was designed correctly.

there is a general rule, and then there are specific situations and execution. sure, dynamic cones have their advantages and disadvantages. so you need to execute over-the-top to overcome the disadvantages.

in my case that way they did it was high efficiency, and overkill with driver surface. you have -4- 15" powered subwoofer drivers per channel in a separate tower for under 40hz, and -4- 11" ceramic matrix woofer passive drivers for 40hz-250hz. this is considerably more driver surface for the mid-bass than any other dynamic cone speaker. and it’s an easy amp load of 97 db, 7 ohm. visitors who hear it exactly comment that it sounds like a planer or stat it’s so seamless and fast. yet it has the weight of a dynamic driver and since it’s an integrated design it is coherent with the lowest octaves from the bass tower too.

http://www.evolutionacoustics.com/loudspeakers/mm-series/mmseven/