Macbook Pro gets slow frequently when connected to external monitor via HDMI cable
This looks like you have a faulty cable and I would look into replacing that immediately. The faulty cable is also causing the performance issues as kernel_task with such a high CPU usage means it is thermal throttling hard.
I've always used the Choetech brand for my USB-C to HDMI but PrimeCables will do as well. Make sure you're getting something rated for 4K 60Hz and check the reviews on whatever you intend to buy. As an aside, I'd look into a USB-C to DP adapter instead as this can result in a better experience with certain monitors.