Fortigate VM ipsec network speed slow issue

Hello everyone,

We have an interesting “challenge” with Fortigate VM ipsec internet speed.

We have VPN Site to Site between Fortigate VM 1 in privet cloud A and Fortigate VM 2 in privet cloud B.

Also, we have windows server 2019 VMs are connected to Fortigate VM 1 in cloud A, they are going to internet via VPN tunnel S2S to Fortigate VM 2 in cloud B.

We have ISP line 1 Gb speed on both clouds.

The issue is that internet speed via VPN tunnel is getting maximum 450 mbps.

We tested a speed test on both Fortigate VMs via WAN port (not via VPN tunnel), and we got speed 1Gb Full-Duplex.

One thing we paid attention that when the speed got to 450 mbps via VPN tunnel, one of CPU core got to 90% usage on Fortigate VM. It looks like Fortigate is using only one core for VPN traffic \ usual traffic (I am not sure).

Fortigate VM 1 - CPU 2 cores, RAM 4G

Fortigate VM 2 - CPU 4 cores, RAM 10G

Do you have any idea?