On request, a test with AV1 codec in webm container served via CloudFront. The first video works fine, but higher resolutions stutter and hang quite frequently.
Video examples courtesy of Elecard, Video compression guru

1280x720 AV1 3.2 mbps 54,4 Mb, reasonable result but quality not exceptional for its weight:

HD video, 1920x1080 AV1 6.8 mbps 116 Mb.  Should play without too many hickups, but this is not the case:

4K video, 3840x2160 AV1 22.7 mbps 390 Mb. Really bad performance.  Other codecs do much better with 4K: