What hardware do you use?

Check using e.g. top for your CPU (nvidia-smi or amd-smi for your GPU) or System Monitor on KDE if any of your resource is being maxed out. If so then most likely you found the culprit.

Regarding what the actual codec is being used you can use ffprobe but anyway what matters if resource bottleneck and thus if you can have hardware acceleration for it.

It's probably worth investigating so that you don't keep on getting video files too big for your computer to handle. I imagine it's something very high resolution with very recent compression. If so, look for something less demanding, e.g. x265 720p and if that's still leading to performance hiccups the older x264 720p or even 480p.

It's rare that the media player itself, e.g. VLC or mpv, actually is the bottleneck.

Run ffprobe on that file and tell us what it really is.

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.

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?

try mplayer?

Try disabling hardware acceleration.