

I had an i5-7200u equipped laptop and I could do AV1 playback, not well but it didn’t do what you described.
Also is it maxing out memory or cpu? I would check btop during playback. Look at CPU usage, memory usage and temps while doing so and report here.
Edit: Something is definitely wrong with your machine, that is abnormal behavior. Maybe it’s overheating, maybe it’s trying to do some sort of gpu decode and has no idea what to do with it as it doesn’t support it. Can you check what encoder mpv is using? I would assume it would failback to CPU.
Just get a thick thermal paste. By and large any thermal paste is an improvement over stock. Thicker pastes like arctic mx6 won’t pump out. I like ic diamond 7 karat but that’s out of stock. (all I have left is a random tube of it that I found in storage)
(I do mean any thernal paste is better than stock. I used arctic ceramique 3 once on my i5-7200u laptop and it was still 3C lower than og psste temps)
When I repasted my i5-1135G7 laptop, I dropped 10C after replacing the paste.
All you really need is a tiny blob, one covering the igpu die, other on the cpu.