Why? Its the masking of adjacent sounds in simple terms. If you have slow driver moving back and forth, it essentially means you are letting the train of signals letting overlap. Either use a faster driver, or use a powerful amp which can control the drivers movement should solve this. Hearing details is only result of not letting frequency masking and temporal masking happening. That’s what I know.
"Fast/slow driver" doesn't make sense in the way you're using it here.
Firstly, sound pressure produced by a speaker driver diaphragm is not proportional to it speed (or rather velocity), but rather to its acceleration. As long as a diaphragm can accelerate quickly, it needn't be moving fast.
Secondly, a powerful amp does not control a driver's movement better than a weaker amp.
Thirdly, frequency masking (or rather spectral masking) and temporal masking are best not thought of as separate phenomena. Masking either occurs or it does not, and whether or not it occurs is dependent on
both the temporal relationship and the spectral relationship between masker and maskee.
Fourthly, hearing details does not result from the absence of masking.
It may help to think of it this way: An effective masker (masking sound) in the presence of a maskee (sound that is masked) sounds
exactly the same as that same masker in the absence of the maskee. That is literally the definition of masking: when it is effective, the maskee is completely suppressed by the auditory system.
In other words, the presence of a maskee (or maskees) cannot affect/suppress perceived details. Indeed, so long as masking is effective, the presence of the maskee(s) by definition does not affect the sound at all.