File copy over VPN

File system overhead is significant for many small files.  Can you move a virtual disk instead?

A similar strategy would be to zip up sections of yor data, and move zip files.

The files should also be broken into a directory tree, if they are not already.    A single folder with many thousand files performs very poorly.

I have zipped a 20GB folder with 7zip, compression ration of 60%.

There are some improvement, its copying at 4MB/s over a VPN running over 100MB dedicated link, I expect faster transfer rate.

I suspect there could be number of factors slowing it down, VPN Encryption, disks etc. Is there anything else I can do to speed this up? 

The next step is to reduce the MTU, on the inside interface, to avoid fragmentation when the inside packet is enveloped inside of the VPN tunnel.  Try a setting of 1300 instead of the default 1512.  For a more detailed analysis of the issue, go to the Cisco support site and search for "IP Fragmentation"

Where  is the change required, Source and destination server?

Inside Interface of the UTM, and  inside interface of the firewall at the other end of the tunnel.