If [the sphere] moves over a pit, a cliff, or other drop-off, it safely descends until it is hovering 10 feet above the ground. Any creature restrained by the sphere moves with it.
When I first read this line, I interpreted that as a way to prevent players from moving the big bad over a 400ft cliff and dropping him. The answer to the question above, however, mentions concentration being a potential reason why the sphere wouldn't reach the bottom.
If you could drop concentration willingly to drop the big bad 400ft, that seems to disprove my reading of it, but if you couldn't, that seems to imply that the sphere moves instantaneously, with no room to lose concentration (while still being "safe").
How fast does the watery sphere move while descending safely, and is releasing concentration before it does possible?