What makes a headphone "slow" or "fast" ?

it's just how quickly the driver reacts to a change in the input signal. fast tends to sound sharper and more revealing of minute details in the signal, and slow sounds muddy and longer decay.

How fast the driver reacts to input signal.
How fast the driver comes to rest upon signal stop.

Slightly exagerrated, a slow fon produces a booom, whereas a fast one a bang! from the same signal.

A driver that sucks in following the input signal will not be able to produce high frequencies, meaning it will sound dull.

A driver not coming to rest after signal stop will produce distortion.

Note that "slow" and "fast" are totally subjective terms, while the actual "speed" of the driver should be easily measurable.

Imho, masking effects of bass-frequencies covering higher frequencies will produce a "slow"-kinda soundsig. Think why there are no bass-heavy phones that have a reputation as a "fast" phone (except phones with boosted bass and treble, which can balance the thing again).

Legends of fast and slow drivers are audiophile pseudo-science, imho.

Also, regardless of what anyone else here says, the truth is that the vast majority of headphones are very, very similar to each other in terms of transient response and stop speed. Most of the speed differences people describe are actually perceptual effects caused by differences in the relative levels of bass and treble in the headphone's frequency response. But this has nothing to do with the actual speed of the driver's movement.

The sad truth is, there is no way to measure the transient performance of headphones. loudspeakers have the MLSSA measurement system which graphically plots loudspeaker transient resonance, over millisecond time intervals.

http://www.mlssa.com/

So... part of me wants to disagree with Piccolo... there very well could be signigficant transient differences between headphones. But, we really do not know, until there is a way to measure it.

I do agree that frequency colorations can greatly influence the minds perception of transient response speed. Grados for example, are perceived to be somewhat "fast" and detailed. But I'm willing to bet IF we had a measurement system to quantify the resonance characteristics, Grados would exhibit resonant artifacts all over the place. Especially the wood ones. Sames goes for the MDRV6.

IMHO at least some of the perceived SA5k "speed" is influenced by its slight treble boost.

It seems you're always the realist.

Its all about perception Perception is truth. Although things CAN exist, even though you can not perceive their existence.

what came first the chicken or the egg

Is it really impossible? I believe the website http://www.headphonesguru.com does transient response measurements of the headphones it reviews.

Its not impossible... Its just that there currently is no standard measurement system in existence (that I am aware of).

That www site says they use an oscilloscope to measure headphone transient response. I am not familiar with an o-scope that measures acoustic sound output relative to time. At the least you would need a mic and an RTA that measures frequency response over time. Most RTAs do not factor in time, merely frequency and amplitude.

Wouldn't size of the driver decide? I know when I was shopping for a Subwoofer I heard an 8" sub and it way faster responding to the sound than the 15 inch sub, But the 15 inch sub rocked the whole building. I bought the 15inch SUB.

I like your logic