I suspect there's another related reason for high kernel_task % CPU: your battery is very low and your charger is underpowered. It seems to throttle your CPU usage to avoid exhausting the battery while charging, and this throttling shows up in the same way as thermal throttling.

I discovered this by accident when I plugged my MBP into my phone charger instead of my laptop charger. I need to mark the cables somehow, or replace the phone charger with a laptop charger just in case...