How to know performance limitation of FG100D

My Internet access performance is bad.

I checked the internet access performance (File downloading ,Google Hangouts call).

When there is no firewall (FG100D), the result is fine.

When there is firewall(FG100D), the result is worse.

So it maybe that  the firewall performance is bad...

I would like to know the way if I have to upgrade the firewall.

Please tell me how to check firewall's perfomance problem....