I have a Macbook Pro 16" and recently purchased a 24" (HD) Dell monitor via HDMI (via a USB-C to HDMI converter)
After about 20m of usage, my mac comes to a crawl and really slows down. It becomes impossible to work on the machine.
Plugging out the monitor fixes the issue in no time and I can see the kernel_task process (in Activity monitor) hit 900% to 1500% of CPU.
I am running macOS Big Sur
It is a 2019 model with Intel Core i9, 16 Gigs of RAM and
the AMD Radeon Pro 5500M 4 GB graphics processor