Very Noticeable Latency Spikes at first Xfinity hop under downstream load even with clean RF/FEC

I’m seeing reproducible severe downstream latency under load. My LAN gateway remains around 0.3 ms and my Hitron CODA56 management interface remains around 1–2 ms, while Comcast’s first routed hop rises from roughly 13 ms into the 200–450 ms range. External destinations such as 1.1.1.1 rise by essentially the same amount.

The issue is directly correlated with downstream load and improves when I reduce download throughput. I reproduced it off-peak as well.

I reset the modem’s FEC counters and tested several minutes of sustained >90 MiB/s downstream traffic. The DOCSIS 3.1 OFDM channel remained locked with ~38–39 dB SNR and 0 uncorrectable. Legacy downstream channels are approximately +7 to +9 dBmV with ~39 dB SNR and essentially no uncorrectables.

Is there any way to get insights into the downstream CMTS/node or connect with a network engineer/tech that could check?