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

Great discussion and lots of good info to consider. At the end of the day, it’s what one hears. I have experienced speakers that are fast but didn’t sound it because of bass overhang attributable to room acoustics. Fix the room and you’ll extract everything the speakers can offer. And also be confident when evaluating new speakers. I have a really good room acoustically and whe I compared new Raidho speakers to my existing Burmesters (both ribbon tweeters) I could really feel the impact of the transients. Not sure I would have before getting my room right. Just my opinion. 

Boy some really odd analogies flying around here. Audiokinesis are you out there somewhere? Your help would be appreciated:)

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.