A small DNS/@Quad9DNS, since it nearly cracked my head finding the solution: I was debugging slow to non-functioning internet connections at my home - concerning SOME sites, by far not all.
Problems were, sites not loading at all, or resources (JS, CSS, images) not loading.

E.g. Amazon website was loading but prime videos were non-functional. Logins to game platforms weren't possible (e.g. bethesda)

First idea was a malfunctioning ethernet at my some, just for the reason that I had done some patching the week-end before. Unwired all new plugs, no change.

Second idea was a problem at my PC, because I was able to load Prime videos with my smartphone. Tested a lot there, reinstalled ethernet drivers, configured a lot, no change.
Turned out that playing videos at PC was breaking mostly because of some JS not loading.

Nearly gave up for that day. When trying to play a game on bethesda, login wasn't possible there. Bethesda site loaded sometimes, sometimes not - when it did, it was very slow.
Pinged http://bethesda.net, very slow, many timeouts.

So third idea was my ISP. Did reconnects, reconfigured my DSL settings to be more robust against noise, no change.

Did a ping from one of my servers (from data center, different ISP), et voilà: I received a different IP address for bethesda in comparison to the one I received at home.
Changed my DNS provider from @Quad9DNS@twitter.com Quad9 to Google: Everythings works fine. But don't stop reading here!

Idea 4 was then @Quad9DNS@twitter.com Quad9 delivered bogus or outdated IPs, but that's not entirely correct: What happened is, that I had selected 9.9.9.9, which is the most secure and privacy-friendly variant of Quad9 - but it does not support ECS. ECS helps in case of big providers/CDNs

by delivering the IP of the server closest to your location. (see: https://en.wikipedia.org/wiki/EDNS_Client_Subnet)
This of course comes with a bit of a loss of privacy, because it has to take into account your location. So perfectly fine for @Quad9DNS@twitter.com Quad9 to NOT do that by default.

So, the result was, that I probably received IPs from content servers, authentication servers etc from the US. That SHOULD still work, but at least it will make things slower...