I need to correct my previous statement: watching it for a bit longer and even trying to optimize things, it looks like hosting Lemmy on that i7 7700 machine can't continue for much longer. I am still trying to figure out whether I can improve things, but it is quite possible that there will be an announcement for moving this instance to a new host soonish.
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.
Having an eye on it... from time to time the monitoring cries too - for me it is often hard to distinguish between a possible server problem or a starlink hickup... as of for right now, i couldn't spot obvious load issues but i have increased some things under the hood in the hope that maybe already helps. :)