If it's AV1, you need both the hardware that can decode it, and the right libraries for it. x264 is not the same as AV1. AV1 requires lots of processing power, that's why you see the slow down. But with the right gfx card (and libs), it can decode it fine. What's your gfx card model exactly?
What hardware do you use?
Is it x264 or AV1? They are two completely different codecs. If it's AV1, you may not have hardware acceleration since it's fairly new. It takes a lot of power to software decode AV1 and you will get dropped frames if the CPU can't keep up.
This right here is your issue, OP. Most likely is AV1 and cant decode it fast enough.
try mplayer?
Does mediainfo give any useful insight?
Try disabling hardware acceleration.
as a noob: on debian or mpv?
In the players. VLC has a setting in preferences, and mpv has a flag to disable on run.
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