Site to site IPsec traffic very slow

Hi everyone !

I'm facing a really strange problem with IPSec VPN. I configured IPSec tunnel FortiGate to FortiGate on different models (40F - 80F and 100F) all of my VPN tunnels are slow and they not reflecting my bandwidth throughput. I'm on FortiOs 7.0.1

For exemple I have :
- FortiGate-80F <-> FortiGate-80F with a bandwidth of 1Gb/1Gb on both sites. Trough my tunnel, I reach with difficulties about 200Mb/200Mb

- FortiGate-100F <-> FortiGate-80F with a bandwidth of 1Gb/1Gb on 100F sites and a bandwidth of 500Mb/500mb. Trough my tunnel, I reach with difficulties about 200Mb/200Mb

- FortiGate-100F - Fortigate-100F with a bandwidth of 500Mb/500Mb on both sites. Trough my tunnel, I reach with difficulties about 200Mb/200Mb

I tried many options to optimise my tunnel but nothing woks. I tried :
- Set correct MTU on WAN Interface and MSS on Firewall rules (for exemple MTU of 1500 and MSS of 1380) --> no change
- Set different encryptions on my tunnels --> no change
- Disabled ipsec-asic and ipsec-hmac --> no change

This slowness on IPSec seems to be the same on every models and on very configurations... Here is for exemple one of my phase1 config

config ipsec phase1-interface
   edit "vpn"
      set interface "wan1"
      set ike-version 2
      set local-gw 1.2.3.4
      set keylife 28800
      set peertype any
      set net-device disable
      set proposal aes128-sha256 aes256-sha256 aes128gcm-prfsha256 aes256gcm-prfsha384 chacha20poly1305-prfsha256
      set dhgrp 19 20
      set nattraversal forced
      set remote-gw 4.3.2.1
      set add-gw-route enable
      set psksecret Secret
   next
end

I really need your help. I don't understand what I'v missed in configuration.

Thanks !