Update: Some YouTube videos don't work on the
mid setting for me,
high seems to be needed. More investigation required!
OK, CPU scaling wasn't the reason for my laptop's excess heat and fan noise after all. The problem was with the GPU.
$ sudo -i # echo dynpm > /sys/class/drm/card0/device/power_method # echo profile > /sys/class/drm/card0/device/power_method # echo mid > /sys/class/drm/card0/device/power_profile
After applying this change there was a marked decrease in fan noise. The GPU temperature has clearly come down.
$ sensors radeon-pci-0100 Adapter: PCI adapter temp1: +66.5°C
$ sensors radeon-pci-0100 Adapter: PCI adapter temp1: +56.5°C