display – MBP 16 becomes slow after a while when connected to external monitor

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.