One CPU core being heavily used by something

Hi all,

My laptop has been plagued by the following issue since new: after some sleep it wakes up really sluggish and requires a reboot.

Today I might have identified what the problem is: ONE core of my Ryzen 7 6800HS (8 cores, 16 threads) is being heavily used by... something. I closed all background apps I could think of but it didn't change. I don't see anything obvious under task manager. I had a look online and there doesn't seem to be an easy way to identify what's causing that.

Any ideas please? Attached a screenshot of task manager while the first core is being used and also my performance tab after a reboot, as you can see the system is normally idle.

Thanks!