Temp. file buffers influence backup speed on the same harddisks (omv 5)

hi there, first post for me so please be gentle.

i have curious problem where the backup speed for image backup is limited (or at least stuck) at 360mbit/s.
normal file copies via smb go up near full gigabit speeds (~970mbit/s). server cpu is not maxed out (more like 40% on a i3-6300t while running the backup) - neither are the clients.

background service setting for clients is unchecked and the image is a normal vhd (non-compressed)

Server is running on docker (openmediavault 5)
speed limits set to 1000mbit/s or off
Clients are Windows 10

maybe some of you fine individuals has any tips for me
thanks for your time - have a good day

turning on the temporary file buffers under advanced put the speed up to saturate the gigabit connection to the server. strange behaviour since the temp storage is on the SAME volume (mergerfs volume) as the final backup destination

Sounds like the VHD writing is a bit inefficient. I’m more focused on the btrfs (or XFS etc.) CoW-file image backup format for now. Maybe someone else can look into it? The code is here