IPSec Tunnel's bandwidth is slower than IPS's circuit

Hello, has anyone experienced ipsec tunnel slower than the IPS? I have two sites and a Fortigate 400F with 148e switch and 1GB fiber circuit at each site. There's ipsec tunnel between them. Ran iperf through the tunnel and got about 36-40MBs bandwidth in both directions. Ran the same test from wan to wan (without tunnel) got similar results. Internet speed test shows 1GB symmetrical at both sites. There's no traffic shaping on either of the firewalls. All firewall policies are disabled for testing. The ISP says everything is fine nothing is wrong. Some internet sites say the ISP could have some traffic shaping on the circuits, not sure if that's true. Any troubling suggestions are greatly appreciated.