What would be an example of decoding-intensive video? my question might come in a bit weird but I'm looking for something that would be considered very CPU-intensive video to decode (for playback I mean). I would like to have some sample of 1080p and 4k videos that would classify to see if my CPU can handle it.

Sorry if this is a bit of off-topic but I'm not sure where else on the internet to ask.

Backstory to prevent XY-problem: HW-accelerated video decoding on my GPU is not working and I'm trying to find if it actually is a problem or if my CPU can handle it fine.
You could try to play 1080p and 2160p videos, possibly 60fps in YouTube. If you right-click on the playback window and click on "Stats for nerds" you will get more information about used video and audio codecs and dropped frames, which can indicate that your hardware may be too slow. Currently the videos are mostly encoded as VP9 codec, but you can change settings to prefer AV1 codec, at https://www.youtube.com/account_playback. However please note that not all videos are available as AV1, but you can find some videos at AV1 Beta Launch Playlist.