To all those bemoaning 8K not running properly, its not your internet, its the browser/codec inefficiency for 8K playback/your CPU, but don't worry about watching it in 8K, the 4K will be largely the same since the source resolution is so low. If it was coming from a 1080p source then maybe it would make a difference, but only if they applied detail with that output res in mind.
Thats literally one of the best upscales and interpolation jobs I've ever seen. Is the original cutscene REALLY at 30fps?
The motion must have been done by hand, I spotted maybe 2 or 3 "artifacts" throughout the first few minutes that were visible in real-time, but they were mostly some ghost-frames which I suspect was left in intentionally to give a sense of speed/a "ninja movement effect". There are tearing artifacts during some really fast complex motion like the fish and the leaves at the beginning but its not noticeable for 99% of people unless you freeze frame it.