I've noticed recently that my 2019 MBP 16' (running latest version of Big Sur) becomes very slow and sluggish after being connected to an external monitor for a certain amount of time (the time varies, sometimes it's 1 hour, 2 hours, 30 mins, etc). For some reason CPU usage goes to 80-90% across all cores when this happens, although the temperature remains steady at ~60C (so I don't think it's CPU throttling or anything like that). Opening Activity Monitor shows that
kernel_task consumes most of that CPU, which I understand it to be part of the OS, although I don't know why it suddenly spikes. I've monitored GPU temps when this happens, and they also seem to be somewhere in the 60s C most of the time. I don't think that's hot enough for throttling, although I don't know for sure.
The issue persists whether I use an USB-C to HDMI dock, or a USB-C to DP cable. Disconnecting the external monitor for a few minutes resolves the issue, but it inevitably happens again once I reconnect it.
I've tried a few things like resetting the SMC (at least I think I have; the OS gives you no confirmation), or connected the power cable and monitor on different sides, but to no avail.
Any solutions to this problem? I'm using a Dell monitor, in case that matters.