How to diagnose why VLC is always using some much more CPU power than it does on YouTube or previously on Windows

I doubt that. MPV is not the same as mpv. Names are case sensitive as are file names.

mpv looks fine.

VLC: try to select vaapi from command line.

LIBVA_DRIVER_NAME=iHD /usr/bin/vlc  --avcodec-hw=vaapi   videofile

you want to see a line that looks like this:

[00007fceecc04ed0] avcodec decoder: Using Intel iHD driver for Intel(R) Gen Graphics - 25.3.4 () for hardware decoding

You can also try to use the older i965 driver instead the newer iHD

LIBVA_DRIVER_NAME=i965  ....

I did install the correct mpv

My processor is 6th gen, I believe the driver is iHD, but I did try i965

~cat /proc/cpuinfo | grep ‘model name’ | head -n 1 1
model name : Intel(R) Core™ i5-6300U CPU @ 2.40GHz

LIBVA_DRIVER_NAME=i965 /usr/bin/vlc --avcodec-hw=vaapi

LIBVA_DRIVER_NAME=i965 /usr/bin/vlc --avcodec-hw=vaapi Downloads/videos/‘Next.js Authentication Master Class - No Libraries (Email⧸Password, OAuth2, etc.).mp4’
VLC media player 3.0.22 Vetinari (revision 3.0.22-0-gf9020c4df0)
[0000563ae32d7520] main libvlc: Running vlc with the default interface. Use ‘cvlc’ to use vlc without interface.

IBVA_DRIVER_NAME=i965 /usr/bin/mpv

LIBVA_DRIVER_NAME=i965 /usr/bin/mpv --hwdec=vaapi Downloads/videos/‘Next.js Authentication Master Class - No Libraries (Email⧸Password, OAuth2, etc.).mp4’
● Video --vid=1 (h264 1920x1080 30 fps) [default]
● Audio --aid=1 --alang=eng (aac 2ch 44100 Hz 128 kbps) [default]
MESA-INTEL: warning: ../src/intel/vulkan/anv_formats.c:834: FINISHME: support more multi-planar formats with DRM modifiers
error: failed to resolve wl_drm_interface(): /lib64/libEGL_mesa.so.0: undefined symbol: wl_drm_interface
failed to open /usr/lib64/dri/hybrid_drv_video.so
Not using hybrid_drv_video.so
Using hardware decoding (vaapi).
AO: [pipewire] 44100Hz stereo 2ch floatp
VO: [gpu] 1920x1080 vaapi[nv12]
AV: 00:04:10 / 01:59:39 (3%) A-V: 0.000 Dropped: 20
Exiting… (Quit)

I have not been able to achieve this