On a congested upstream, DOCSIS unfortunately seems to behave like this due to contention for and the insufficient granularity of upsteam transmission timeslots.

This effectively means that even if your share of the total upstream link of a network "collision domain" (shared coaxial medium really) is low (i.e you're using less than 1/n and accordingly should not be experiencing queueing in the modem), you might be seeing latency spikes due to having to compete and wait for transmission timeslots.

My local DOCSIS link is experiencing anything between 0 and 60 milliseconds of latency which I suspect is mostly due to this (since it's inversely correlated to upstream bandwith).

Seems like everyone with any cable ISP has this problem. It could be a modem problem, but it's happened with a variety of modems. It could be an ISP problem, but both Comcast and Spectrum users see the same thing in my experience.

I'd be interested in hearing counterexamples, but I imagine that people with good experiences are on DSL or Fiber.

I can ping 1.1.1.1 right now at 12-15ms, or Google at around 20ms, via Comcast. Less than 10ms to either one on Optimum. But it's 25-30ms to ping Optimum from Comcast.