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

All speakers have mass and inertia, because they are mechanical devices pushing air. This means the "thing" in the speaker that moves the air will never respond instantaneously to what the amplifier throws at it (i.e. be fast).

Electrostatics is one technology that is 'faster' than most.

Large cone speakers (woofers) driven by electromagnetic coils are slow because the cones are heavy and electromagnets have electrical inertia. Electromagnets resist any signal feed to them from an amplifier and they return a signal back to the amplifier (back-EMF).

Low impedance output stages on amplifiers are important because they help 'dampen' the back-EMF, effectively stiffening the cone speaker. 

Tweeters and electrostatic panels tend to have light mass - they respond faster than woofers (bass speakers). Hence why Martin Logans can sound like the bass is slower than the panel.

Some speaker designers used to put the tweeters physically behind the plane of the woofer to try and accomodate the slower response of the woofer. Perhaps the B&W 800's do this?

Any two or three way speaker will struggle with phasing. Two way speakers often sound cleaner because of less speaker phase issues and the cross-over networks are simpler. Cross-over networks that use inductors and capacitors can potentially introduce phasing issues.

Quad electrostatics always sounded great because they were basically a single cone speaker. Bass was not great though. It was interesting playing Telarc's 1812 CD through Quad ESL 2905's.
 


 

There are many different factors that can make a speaker sound “faster,” but in my opinion, the biggest factor is the class bias of the amp.  I’ve made this analogy before.  A class B bias amp is like a dragster sitting on the starting line with the engine at idle.  When they get the green light, they floor the throttle and are off toward the finish line.  A class A bias amp is a like a top fuel dragster.  The throttle is wide open and they pop the clutch getting off the line SOOO much quicker.  Class A bias amps are always wide open, just waiting to reproduce the musical note, hence a faster response time.

I believe it is all things that come together to have a fast set of speakers. Well designed paper drivers in a well design enclosure . With as high end crossovers as you can afford , with an amplifier with the appropriate taps , bi amplified . I think fast is what we are all trying to achieve . A speaker doing precisely what it is told . 

@obelisk- "Some speaker designers used to put the tweeters physically behind the plane of the woofer to try and accomodate the slower response of the woofer." No- The physical time alignment, in multi-driver systems, has nothing to do with the relative speed of the drivers. Rather: to get the acoustic centers(actual sound source/voice coil) of the drivers, in the same plane/in phase. https://en.wikipedia.org/wiki/Loudspeaker_time_alignment