I've learned a strange lesson through repeated experiences: After checking all other known variables, the question must be asked:
"Ok, do you have an Alexa?" (or Google nest).
A surprising amount of the time the answer is "Oh yes!" and by unplugging it, we find it's as though we had performed a technological exorcism. All the bandwidth demons are gone.
It's not only a bandwidth thing. Google Nest and Alexa completely crowd out other devices, and do something to the router where you can't even get an IP without powercycling it. I don't mean to be vulgar, but Nest and Alexa seem to gang rape everything on the network.
I haven't taken it as far as bringing one home and watching it's activity, to spy on the spy. One thing I can confirm, though, is that the bandwidth issues do not always come from Alexa / Google using the internet.
I've seen a lot of WISPY people on hackernews, so I thought I'd throw this question out there.