As it turns out, my ISP was just partly the problem.
My friend over from diemo.best had the idea to just disable my freifunk router to see if that helps. spoiler: it did.
Now why does it help?
Vodafone Germany fucked up their ipv6 routing and therefore my freifunk router took over the ipv6 handling,which slowed down my internet speed significantly ( predictably to 16Mbit/s ).
As of now, vodafone still hasn´t fixed that. I´ll have to try and disable ipv6 routing in my freifunk router and see if that helps – because i still want to use the router as my guest wifi.
So, if you run into the same problem, and you are sure its not on their part (by checking your ISP´s speedtest, which sometimes shows the speed to your router and the network internal speed seperately) – try disabling some of your equipment, especially routers and see if that helps.