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.