I dunno. But if AirPlay works by sending your screen over wifi, it might be a bandwidth issue. Streaming video from youtube to your phone and then from your phone to a TV can be quite intensive, especially with modern phones being 1080p or more, which takes more bandwidth to stream to a TV. I’ve had this problem on a slow wifi on Android, and there’s not much you can do to fix it besides upgrading your router.
However, it doesn’t mean that’s exactly what is happening here. Certainly someone could be throttling video quality on purpose and we wouldn’t really have a way to prove it. With how shady and monopolistic big tech companies are, I wouldn’t put it past any of them (google and apple)