Fortigate 101F slow internet speed

Normally our internet speed at company should be 500 mbit/s corporate internet.
Here is my configuration SO: GPON router LAN/WAN > Forti WAN Port > PORT1 > HP managed switch > AP devices.
But We get 70-80 mbit/s internet speed.
Because of this installation ( which I think is something wrong ) I created a DMZ Host on Gpon Router