In regards to the “mysterious” Cadence cooldown, it is a very straightforward mechanic implemented to avoid issues with WPS skills that hit multiple times. I vaguely recall that there may also have been issues with passthrough projectiles on ranged weapons and Cadence, but it’s been I somewhere around 4-5 years when this mechanic was introduced…
In any case, the internal cooldown is a fraction of a second to prevent multiple charges of Cadence to build. Before the cooldown existed, Amarasta’s Quick cut could, for example, proc Cadence incredibly often.
You may occasionally run into this cooldown at extremely high attack speeds, but it’s going to have little to no impact over time.