[Spyro]

At least from my inquiry I think you guys are thinking way to deep here.
 
Let's take 2 different IEM's.  ER4 and W3 (or IE8).   Don't even want to get into to which is better or more accurate as that is not the issue at hand.   The ER4 sounds very quick in attack, maybe too much to maintain naturalness while the later two seem NOT necessarily slow but slower in attack and decay where the presentation sounds like in a bigger room or hall and NOT so directly wired to the brain which can be fatiguing.
 
So what is it about the design or tuning that makes this difference?  And, yes, I have to assume it is intentional and not arbitrary.
 
Maybe an even easier example is UM3X versus W3.   Both triple drivers with about the same amount of bass.  UM3X is a stage monitor and sounds directly wired to the brain with little "space" where the W3 sounds about 10 rows back and generally more pleasurable to my ears.

 

[bigshot]

I think your use of the words quick and slow are the problems here. There is no way for them to alter the attack or sustain, so you have to be hearing something else. My guess is that one is louder than the other.

 

[xnor]

I have no idea how the W3 sounds or measures, but the other earphones you mentioned all have a completely different frequency response with the ER4 having lots more emphasis on treble, which will give the impression of faster sound.
 
 
edit: Found some measurements. Both W3 and UM3X have similar spectral decay, but the W3 has a ~5 dB peak at 2 kHz plus there are other huge differences in the frequency response (for example W3 has a lot flatter and less bass with foam tips).

 

[StanD]

There's a big difference bet the attack or decay of a waveform vs.the envelope of a musical note which some folks are using words that imply that the release of the envelope is being lengthened by IEMs/cans. The release cannot be changed but the perception of the attack can be affected to a degree the gives a better or lesser sense of impact to percussion instruments. In no way is the attack going to change from plucked to bowed or visa versa, for that you need a sound processor.

 

[Spyro]

   
edit: Found some measurements. Both W3 and UM3X have similar spectral decay, but the W3 has a ~5 dB peak at 2 kHz plus there are other huge differences in the frequency response (for example W3 has a lot flatter and less bass with foam tips).

What can you find on the W4?  In a long long list of IEM's I have owned I find bass about the best I have ever heard.  There is a very slight lower mid bump for the sole purpose of making vocals more lush but the overall bass texture and depth is outstanding.

 

[xnor]

Couldn't find anything really special. A bass peak centered at ~100 Hz. I guess not playing deep bass at the same or even higher SPL than the mid-bass, like many earphones do, makes the bass sound more natural. Distortion is low.