Can you describe your setup in full? OS, storage setup, network setup, hardware specs, etc. Your mention of mergefs means that the iperf results probably does not accurately represent a real world scenario.
Jellyfin 10.10.5 (Docker)
Ubuntu 24.04 LTS
Intel i3 12100
Intel Arc A380
OS drive - SK Hynix P41 1TB
Storage
3x WD Red Pro 6TB CMR in RAIDZ1 (JF Library)