High latency under load, even with a traffic shaper

This issue started happening sometime in July last month. Btw, I'm using comcast's modem (Cisco DPC3941T) in bridge mode.

For whatever reason, running a dslreports.com speedtest with a sufficiently high number of concurrent connections nets me lower latencies under load, but running with a lower amount of parallel connections nets me higher latencies under load.

I get frequent 300ms latency spikes throughout the download portion of the test. Shaping to half my plan's speeds (25Mbit to 13Mbit), I still get frequent 100ms spikes. Increasing the # of streams for the test from '8' to '16' gives me a steady 20-30ms with an initial 100ms spike. I confirmed these results through my router with traceroute.

Without my router's shaper, same issue but somewhat less severe:
with 2 streams: initial and occasional 400ms spikes, usually stays at around 50-80ms

with 8 streams: initial and occasional 400ms spikes, usually stays at around 40-60ms

with 16 streams: initial and occasional 400ms+ spikes, usually stays at around 40-50ms

With no/minimal activity on the network, I easily get 16-22ms ping without any sort of spike.

I've checked my own equipment/connections so I don't believe this issue is on my end. Also tried restarting the modem a few times. Online gaming is impossible if anyone tries to watch videos or download things.