As far as I can tell, my issues are all resolved now…
That is to say, changing my 2.4GHz to Channel 6 appears to have fixed everything. Calibrates in seconds again, scans materials quickly, back to its old self. The issue remains, why did these things stop working well on Channel 9 as it has for the past year or so? Over the course of all of this I, of course, checked the channel and still found it to be the cleanest channel for me. 6, on the other hand, is pretty noisy (a lot of wireless routers default to 6 and most people don’t change that), yet it appears to be working perfectly.
As far as I’m concerned, this case is closed. But I suggest Support should still examine my logs and figure out what’s happening with Channel 9. I’ll even accept it if they say “Your router doesn’t broadcast on Channel 9 well anymore.” but that seems unlikely since every other device in every room on every floor works flawlessly.