Performance problem VPN IPSEC on fiber optic support

Hello and thank you for taking the time to read my problem, and sorry for my poor level in english!

Here is a description of my network infrastructure : At an iaas host, I have a forti vm cluster managed by a FortiManager. I mounted several vpn ipsec on this infrastructure through adsl and sdsl links without problems. IP telephony and data transit through these tunnels.

I've been trying for a while to do the same thing on a fiber optic link behind a FortiGate 50E but I'm having problems. The fiber optic link is good: 400 MB download and more than 200 MB upload. No worries on this side. I know that the speed is reduced behind the firewall in ipsec.The vpn tunnel is up.

when i want to access a youtube page for example, the page loads very very slowly. It looks like a 2 MB adsl link. The phone connection is very degraded and when the internet traffic increases, the call is cut off.

Has anyone ever had a similar problem? What can I give you as information to try to solve the problem?

Regards.