Same for me!
I have Vodafone Cable 1 Gbit in Germany. Videos have always been loading slow at the start, but now it takes minutes to buffer a short video in the shop and all the Graphics are loading very slow.
No issues with downloads, i usually get more than 100 Mbyte/sec downloading Games from Steam.
Hope this will be fixed in the Future, because it makes Steam almost unusable for me :/

Same on my end. Only thing different is, that it only occurs in the Steam Client and not while on the Steam Shop via a webbrowser.

Are you using also Windows? I am on Windows 11 and I think it is a Windows issue.
Like OP, I tried litarally everything that is suggested and then saw in taskmanager, that Windows put several of Steamwebhelper.exe into "efficency mode".
Rightclicked them and deactivated those efficency modes, voila, everything running smooth again...
So I searched for a permanent solution to deactivate those efficency modes, stumbled upon "Process Lasso" and I got rid of it. Well for almost two weeks. Now the problem occurs again and I don't know how to solve it and I am fed up with it really.

Edit:
Forget it, it is not only on the Steam Client but also on different webbrowsers (Firefox, Edge)

I have the same problem. Some days it works, other days it doesn't.

Do you guys have hardware acceleration disabled, I have this issue but it seems to be related to that setting on my end.