elim_garak I'm normally loathe to suggest it, but have you tried overclocking the board?
It seems one of the causes for the poor performance is "local warming" despite being winter season.
My rpi4 has an idle temperature of 55'C. By playing the AV1 file the rpi4 reaches 85'C and gets throttled with both Arm frequency capped and Soft temperature limit active.
I discovered this as I went to overclock it; which I still did. But as expected the same thing happened. At first it seemed the playthrough was smoother but then the rpi4 started making noises I've never heard before. And I don't even have a fan. Nor heatsink. Just using the official case with the top off.
FWIW these were my settings:
over_voltage=6
arm_freq=2000
gpu_freq=700
I've been looking into a case and will probably go with the Argon One which is both a massive heatsink and includes a configurable fan. I'll have to endure the fan for AV1 playback. Hopefully it won't be too annoying.
OTOH maybe the smarter alternative would be just to invest in a rpi5 instead of burning 25£ on an additional case.
Your bootloader version does pre-date automatic NUMA settings and SDRAM_BANKLOW will be ignored.
However looks like you are on a Pi4, so SDRAM_BANKLOW=3 was the default so it may not be critical, but it may be worth updating that to be sure.
I'm using PINN with Raspbian and LE. But I haven't updated Raspbian to Bookworm.
But when booting to LE isn't the bootloader firmware tied to the LE version? I'm not sure how to update it since that is the latest version shown to me.
It might be best to try and find a file that plays well on one of the systems but not the other (something where you can check a/v sync is accurate throughout). If the answer is "struggles on both", then it might just be slightly different thresholds in kodi makes one appear better (but really you can't easily judge which is better when both fail to keep up).
After discovering my rpi4 is being throttled I'm wondering if the effect of one player better than the other wasn't just chance due to throttling effects.