is the free vpn service the wireguard one? if yes and it is slow, than it won’t be any faster when using your own domain and exposing the server directly to the internet, because wireguard should be as fast as any direct connection. if it is not the wireguard vpn from the fritz box i’d recommend switching to it. this can be done by tge server jellyfin is running on if your box does not support wireguard.