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

Obelisk, rodman is correct. The drivers have to be phase/time aligned. Sound from each individual driver has to reach your ears at exactly the same time. The Dalquist DQ-10 was the first speaker to try and deal with this. 
It is all about transient response, the ability to start and stop fast without ringing.

Hold on Andy. That is a mistake. Yes they are all transducers but their ability to function in air is quite different. ESLs and horns are a better impedance match to air and transmit sound waves more efficiently. Dynamic drivers have to work harder to get the same results. ESLs work entirely differently than conventional speakers. First, there are no magnets. ESLs are capacitors conventional drivers are not and represent an entirely different load to the amplifier. Designed correctly their transient response is superior to conventional drivers because the moving system has far less inertia and is a better impedance match to air.
Planar speakers are somewhere in between. IMHO in spite of the compromises you have to make ESLs rule as long as you can make them. I have personally not heard a conventional speaker sound as convincing. Some say well designed horns can do it. But I am still waiting to hear one that does.

OK, you're right.  Opposite attracts :-)

By the way, horns use conventional drivers so they are probably the same.

Time cohesion has never been proven. Most speakers are not time cohesive and nobody seems to care or notice. 

Most speakers are not time cohesive and nobody seems to care or notice.

Just because nobody seems to care does not mean it does not matter.  Just because if something has not been proven, it does not mean that it's not true.  

I agree with most of the theories above for sure.  I also agree that the room acoustics have a lot to do with frequency dispersion and speed. To much absorption and or diffusion will have a profound effect on the fastest speaker production.
It is unfortunate fact for all of this as it really difficult to find the perfect combination without serious experimentation and or expense. I am fortunate to own full ribbon driver speakers and have had all types before them......they are easily the fastest I have experienced because of driver mass and height of the panel allowing driver radiation accuracy. In addition, I have experimented with acoustic treatments extensively (both absorption and diffusion) and feel they do pretty well with minimal treatment accept for my room's obvious modes. Nothing will be perfect so you have to choose the combination that pleases you most in the overall selection of components, speakers, and room acoustics. Everyone has different thoughts in perceived musical reproduction.

Best of luck,
gwalt