i don't use mergerfs... use unionfs...
but i can confirm problems with speeds from hetzner helsinki...
spent a week trying to get answers/fix them.... ended up just moving to a german server in the end....
i always read about peering problems to the u.s..... but ive never had any at all even to the west coast i get iperfs of 22-23 MiB/s a sec solid and steady.... way more than even 4k needs....
i will say that i do get crap results before i make two tweaks.... i update the kernel, change congestion to BBR and increase a bunch of the tcp settings.... YMMV.... but it almost doubles my throughput.... and again ive never had any peering issues since i made those tweaks a couple years ago... and never seen the need for cloudflare etc...
helsinki seemed like a lost cause though... even after the tweaks i'd see drops to like 300k for an hour
: ) : (
perfect again in falkenstein now though....