After looking into webm a bit more, that's just a container. It's the content and its 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 operating 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.