When the damping is low then the diaphragm will not immediately stop moving after you stop the input signal. Even the tiny diaphragm has a mass that wants to keep moving in the same direction. I'm no IEM expert, but I've never seen decay problems with IEMs.
I think "decay" problems are once again a simple frequency response problem. Boosted sub-bass will give you that "everything feels so slow" feeling. Boosted treble or some peak) will make everything/certain instruments appear "faster". The ER4 has no boosted sub-bass and boosted treble, so the perfect "fast" earphone.
Perceived speed has extremely little to do with driver speed.
Edited by xnor - 12/23/13 at 1:55pm