slow degraded throughput on ipsec tunnel between fg-80C units

Good day.

Kindly assist with the following "issue" I have.

Short story, investigating slow performance on customer site. They have a Ubiquiti Af-5X link between two offices. The Air fiber link runs at about 130Mbps. I have tested with my Linux micro server at each end, and confirmed the Link is 100% fine.

Site A:

Fortigate 80C

Firmware: v5.6.5 build1600 (GA)

Site B:

Fortigate 80C

Firmware: v5.6.5 build1600 (GA)

Problem:

When I do a copy from one Windows server on Site A to another Windows server on Site B, via the IPSEC tunnel between the two firewalls, I get nothing more than 2.6 M/sec, basically about 25% utilization of the link. Doing the same copy test outside of this IPSEC tunnel between two server, I get 8-10 M/sec speed, which is fine.

Firewalls are just there to inter connect the two offices. 

Phase 1 Proposal (IPSEC Tunnel):

Algorithms: AES256-SHA256

Diffie-Hellman Group: 5 ( I tried from 21 to 1, MARGINAL difference )

Authentication Method: Pre-shared Key, IKE version 2

Even on the Ubiquiti equipment, it shows the link bandwidth, and then the traffic throughput, you can clearly see the IPSEC tunnel uses only 25% max of the link. 

Please assist with some help, it would be much appreciated.