Slow transfer with VHD

I am using UrBackup Server version 2.4.13 which runs on Linux Debian and client 2.4.10 - 2.4.11 on Windows. Mass memory is connected to the Server using the NFS protocol, backups are stored there. So far, I have used compressed VHD and the transfet is in the range of 200 - 300 Mbps, but when I want to switch to VHD the transfer does not exceed 6 Mbps. Local network backup speed: -
I have tried on several clients and it always does not exceed 6 Mbps. What could be the problem?

Any idea?
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