Horizontal black lines? Dunno what that could be about, we render to a <canvas> element which is resized to match the source video and then resized again to match the window with CSS.
An interesting and unpleasant side-effect of rendering to canvas: it bypasses video autoplay blocking.
I don't think the performance would be any worse than the <video> tag. The only exception would be browser bugs. It definitely sounds like the black bars are a browser rendering bug given it's fine when recorded.
It really only matters in long form content where nothing else on the page is changing though.
Did you not just describe at least 99% of all web video?