Greetings,
i'm another user affected by the choppy youtube playing (and very slow Steam downloads) since the web scan feature introduction. I've noticed that the suggested workaround implies rebooting the pc after disabling the WS, but in my case (ZAISS + Firefox here) rebooting isn't required: for Youtube, just refresh the page after disabling it, and for Steam just pause and then resume the download. Hope it will help other users with the same issue to save time.