Tearing has nothing to do with the window manger but is more a result of how modern graphics cards work in combination with the X11 server. You need to use a compositing manager like picom, if not inbuilt into the DE. Another way is adding "TearFree true" to the section of your graphics card in xorg.conf, this might or might not work and will lower drawing performance though, also might only work with AMD and intel GPUs, I am not sure.
video players are more efficent than browsers because most browser packages floating around in various distributions still don't support GPU-accelerated decoding and it's all done in-CPU. Even if browsers moved to support it in linux in recent times, it's still often fickle and doesn't work right. Also youtube might serve up videos in formats that your GPU can't decode, sometimes a specific format needs to be forced, that is possible with parameters to mpv.