Hi,
I did a little research, as I'm also suffering form the 20MB/s max transfer rate when taking backups of VMs on remote Hosts.
Only thing I could find was this in the documentation center: VMware vSphere 5.1
It says there are NFC Session Connection Limits:
Limited by a transfer buffer for all NFC connections, enforced by the host; the sum of all NFC connection buffers to an ESXi host cannot exceed 32MB.
52 connections through vCenter Server, including the above per-host limit.
Regards