Second router drastically throttling data

[Kisekki]

I am having problems with my internet speed being throttled by my second router, when I am connected to my primary router I get speeds of up to 640 mbps, but after it goes through my extender that speed dips down to maximum 30 mbps. Both routers are rated the same speed and protocol. thanks.

EDIT: Sorry! just read the rules, my routers are both apple airport extremes and my adapter is a TP-LINK T2U (I don't think this is the problem, I have tried this on multiple devices and the speed is the same)

 

[hang-the-9]

Are the routers linked with Ethernet or WiFi? If WiFi, that is what is causing the issue, it pretty much halves the WiFi speeds due to the fact that's it's linking to the other router.