On the release blog for 21.12 I saw 2 users talking about devastated rendering performance, but am I the only one who has had playback performance destroyed? I haven't tried rendering yet so I don't know if it's a match for what they were talking about.
For now I have rolled back to 21.08.3, where playback is rock solid 60fps on any giant clip as expected of my machine. I never used movit or anything, nvidia drivers automatically activate in the 'setup wizard'. This bad performance is back to what it was back in 2019ish.
I'm on mainline archlinux comparing 21.12.2 to 21.08.3 playing back simple h264s. In the bad 21.12 version not even setting playback to 270p reaches 60fps.
Since I have a satisfactory workaround for the time being this is essentially just a poll to see if I'm the only one affected by this. Similar reports include ctrl+f 'hardware' on https://kdenlive.org/en/2021/12/kdenlive-21-12-is-out/ and a ghost user https://www.reddit.com/r/kdenlive/comme ... onths_ago/

If not done already try with AppImage or Flatpak 21.12.2. Does the performance issue still happen with these official packages?

Good thinking, the appimage works fine for 21.12.2

Not sure what happened, perhaps performance actually was normal, or perhaps I only tested just a few seconds, but this issue is actually still present.
Poor playback in all the available appimages (21.12 & 22.04) and normal archlinux packages.
I used the archlinux archive to again roll back to 21.08, where performance is as desired.
I also tried comparing using movit, but I just crash with movit enabled.

Where might I start looking for clues?
There's nothing in console output that looks actually notable, and nothing there shared between the versions that are crippled. Not by eyeballing it, at least.