Part A normally takes somewhat longer to dissolve because it contains the Dimezone-S. Phenidones are harder to dissolve than say metol (for example). They are easier to dissolve in alkaline solutions though. This is why part A includes the borate/metaborate along with the Dimezone-S. Then dissolving part B brings the pH down to target.
I'm not clear on what type of floaters people are referring to. Generally there will always be some undissolved particles and junk, which is of no consequence. Regarding XTOL specifically, many people (including me) have long noticed that after some storage time post mixing the unused stock solution seems to develop some floating whispy sort of clouds (also of no consequence) but this isn't a recent phenomenon. I've observed these whispy floaters in XTOL since who knows when. Using distilled water seems to help to some degree but they aren't worth worrying about.