It's common to find that the buffers in front of the bottleneck last-mile link are sized for the highest tier of service regardless of what speed you're actually subscribed for. DSLReports has the only browser-based speed test that measures bufferbloat, and they have extensive data from that testing: https://www.dslreports.com/speedtest/results/bufferbloat?up=...

Your modem and the box on the other end of that bottleneck link are probably buffering far more than is reasonable. There's simply no reason for a cable modem to ever have in excess of 1s worth of backlog.