CPU Frequency Drops to 0.8 GHz on playback

[atothegreen]

I'd been troubleshooting a CPU issue for the last 6+ months, and finally just went to grab the Intel PowerGadget that everyone's been using because of the MBP i9 fiasco. Lo and behold, on my 2015 machine, playback immediately causes a drop from an above-clock core speed to, apparently, the bare minimum operating frequency for an Intel processor.

Annotated screenshot attached. CPU Usage in the PT window hovers around 14% when the session is open, routinely creeps up to 45%, and then completely tanks out with a CPU spike. Weirdly, those spikes don't show up in PowerGadget at all.

Tried this on a separate session with 12 audio tracks and not a single plugin – same thing occurs.

So... any thoughts?