It’s been an ongoing issue for many people since June. We have had replies saying it is a known issue and is being worked on. Updates have been few and far between and the issue definitely still exists.
By the sounds of things on this form it’s a nationwide problem. The problem is any YouTube video that’s 2160 (4K) @ 60fps and 1080p (HD) @ 60fps the video playback lags (And yes I’m on the highest speed: 500/38 Mb) so the work around is downscale to 720p in the youtube options or just watch on another device.
I know the workaround is to drop quality to 720p but having 350mb broadband this really shouldn’t have to be done. I can watch 4K and 1080p no problem on every other device. Must be a limitation of Youtube app within the V6 box