iPhone X 4K60p is 50Mbits/sec, 4K30p is 30Mbits/sec, no way that it can stream from iCloud to Apple TV4K even with fiber without lags. Imagine 10.000 user streaming at the same time, it can even get lags on a local wifi or ethernet cable with file acces to mac.
You tube stream 4K30p BUT compressed with a bad quality may be 8Mbits/sec.
The only real way to get real 4K60p is to download the video file to local disk or fast usb stick and plug that into the 4K TV.