Fortigate and speed wan

Hi sasaki11,

Thanks for using Fortinet Community. We’ll look to get you an answer or help. In the meantime, if anyone else has any advice, feel free to respond.

​@sasaki11 
Connecting a higher-throughput WAN than your FortiGate’s rated UTP capacity will not disable security protection automatically.

FortiGate will continue inspecting all traffic, which causes system hardware utilization to skyrocket beyond its designed limits.
This overload will lead to increased network latency, packet drops, and slow administrative responsiveness as traffic queues build up.

If resource limits are pushed too far, the unit will enter Conserve Mode, which can drop new connections or fail-open/fail-close sessions depending on your settings.
If fail-open has been configured, traffic will be allowed to pass without inspection.
If fail-close is configured, new packets that arrive will be dropped. This is the side effect and how traffic is disrupted.
Basically you will see a performance degradation.
There are a couple of articles to help you prevent this as well:
Memory related:
Technical Tip: Free up memory to avoid conserve mode | Community
CPU related:
Troubleshooting Tip: How to identify high CPU usage caused by traffic volume (WAD/IPS) | Community