I have this effect also sometimes, but not always. It's also different from client to client. With Thunder on Android it's most of the time really fast. On the other hand I'm using sometimes Voyager, where it's a mix between slow and fast. I use the website only in rare case's. But when I do, it was always fast.
Reading about the Gateway error. Have you possibly subscribed to any feddit.de communities? The instance is no longer accessible, not at all on the web, I can't say what it looks like with the API, but try to remove subscribed feddit.de communities and see if it is better.
Edit: I did find some communities I was subscribed too that are on instances that seem to have gone down - [email protected]l, [email protected], and [email protected] (which gives a malware warning from ublock Origin when I open it). So I'll see if that helps.
Weirdly, I can't unsubscribe from [email protected] - it shows up on my subscription list, but when I open it I get Subscribe button. Tried subscribing and unsubscribing but that didn't work.
Maybe you can try to switch your browser to the so called "porn mode" or "private window" and login there. This will decouple your current session and you don't have to clear the cache. If the community is also visible here, send a mail to the admin and ask for help. He is a nice guy.