Ok the cpupower results confirmed that performance mode is not active. The acpi-cpufreq driver is the older ACPI CPU frequency driver. I guess you have an older AMD CPU? You can use inxi -Cx to get the details.
Try setting performance mode explicitly…
`sudo cpupower frequency-set -g performance’
…and test the video playback / gaming behaviour again. Note that the CPU will then run at it’s maximum frequency all the time, so will run hotter. No dynamic scaling as such. You can also do this via System Settings > Power Management > Energy Saving, switching the power profile as required.
BTW, as you’re using KDE Plasma X11, examine System Settings > Display and Monitor > Compositor, and enable “Allow tearing” as stutter is compounded by compositor frame timing. See if that makes a difference here.