SpeedTest with FortiGate and Firefox

[Bipath]

Hello Community, 

First of all, we're using the 500D with v5.6.2 build1486 (GA) in a high available management cluster in our company. 

Few days ago we've got an update at our fiber internet connection to 100/100 (D/U). If I connect my laptop (and each other) in front of the firewall and start the speed test with Firefox, we're got full 100/100 speed but if I connect my laptop directly behind the firewall and start the speed test with firefox again, I've got 100/30 (D/U) speed. 

After this text above in the datacenter, I've started Google Chrome and start the speed test again, and ... voilà, I've got 100/100 (behind the firewall) in my office room. 

Anyone can explain this problem or has the same trouble? I use the latest version is Firefox always. 

Best Regards

Bipath