I was running a Handbrake re-encode yesterday on my Mac Studio with M1 Ultra and it pegged all of the CPU cores at 100% and ran for about 5.5 hours to complete. After a couple of hours, the CPUs hit 78-degrees C and everything else was considerably cooler. The fans never spun up past their idle speed of around 1,350 rpms and the Mac Studio remained quiet. Feeling the enclosure, I found that there was just enough warmth to be detectable and this is the first time that I’ve ever felt the enclosure not feeling cool to the touch.
So, the system so far has maintained very reasonable temps, even when under a considerable load over a long duration. It appears that it has a very well-designed cooling system.