What is wrong with YouTube on Sky?

[Grumble4162]

For the last week or two YouTube has been a problem when running on my UHD Sky Q box. Thumbnails don't show, every video freezes and it's all a bit rubbish. I have fast internet and if I just use my TV it works flawlessly. I've updated my apps on the Sky box but that didn't help. 

I know I can just use my TV but I have a Sky box that has a YouTube app - it should work!