INTERNET CONNECTION ISSUE IN ZYXEL USG FLEX 100

We have 2 ISP providers in our office but they have a different speed capacities. Our primary is 100Mbps and then the secondary ISP is 650Mpbs which is connected to TPLink TL-R605 SafeStream Gigabit Multi-WAN going to Zyxel USG Flex 100 Firewall and from the Zyxel USG Flex 100 Firewall going to our network switches.

If when we try to speed test on our network with the Zyxel USG Flex 100 Firewall, it only gives us 300Mbps to 400+ Mbps. But if we try to speed test directly from the modem of our ISP providers that has a speed capacity of 650Mbps using Ookla apps, it gives us the accurate speed but if we will use the google chrome web browser, it only gives us 170Mbps to 400Mps speed capacity.


My question is, does the Zyxel USG Flex 100 limit or filter it that's why the speed result of our internet connection is lower than the declared speed capacity from our ISP providers? 

Can anyone give an idea or any advice that is going to test or try to solve this kind of issue? All the comments & suggestions are highly acknowledged & appreciated. Thank you very much for the help in advance.