Not just you. I've seen this quite often lately, usually with webm files. I've extracted the video from the archive, and it plays perfectly in VLC, but ren'py can barely render two frames per second.
I've only seen the devs of another game mention it in passing, but it seems to be something about the encoding that doesn't mesh well with ren'py, so a webm video needs specific settings to run smoothly in ren'py.
After looking into webm a bit more, that's just a container. It's the content and it's encoding that matters, video and audio. Some codecs and settings flat out don't work with ren-py, some can't utilize hardware rendering, and some might cause issues on certain operrating systems. Too much compression can also cause performance issues.
I would encourage @RedRabbit3784 to consult the ren'py documentation and forums, and to make sure all videos are fully compatible and hardware-renderable via ren'py.
