I like your analogy; some coticules are more finicky than others. In other words: it takes more practice to coax a great edge out of them.
However, I never noticed any distinct causality between soft and fast, and hard and slow. I judge hardness in coticules by how readily they release slurry (I'm very sceptical about the fingernail scratching test). One determining factor (definitely not the only one, though) for the functionality of coticules and Belgian Blue Whetstones is the density of the garnet distribution. To me it seems logical that this has a much higher level of impact on the functionality than the hardness of the binder of the coticule.
I have a relatively hard old coticule which is quite slow on slurry and water. I also have a recently mined Les Latneuses combination coticule on which the "hybrid" side is quite hard. Yet, it is not slow by any means; not very fast either (slower than the cream coloured side), but rather what I would call medium speed on slurry and relatively slow on water.
I also have a relatively soft coticule which is roughly as as fast overall as the Les Latneuses "hybrid". As far as I have been able to detect there is no clear correlation between hardness and speed concerning coticules.
Regarding the initial post: I also have trouble descerning between a great coticule edge and a great Thüringer edge. For good measure, add a great Blue Belgian edge to that.