@AndersV 

Hello, sorry to hear the Wi-Fi speed is not as good as expected, please help verify some details and we will try to fix it together:

1. Ensure the C9 firmware is up-to-date. If not, update it first and reconfigure it from scratch.

2. How did you test this Wi-Fi speed via the router? Did you disconnect all the other devices from it and only leave one laptop or phone performing the test? It is suggested to change a new SSID and password and connect only one phone or laptop to perform the speedtest 3 times.

3. How about the wireless negotiation speed on that device? Is it connected to 5GHz or 2.4GHz? If the link speed is not as good as expected, please try another wireless device with more powerful wireless cards.

4. Are you testing the speed by speedtest.net? Please try to download the Speedtest app on your phone or laptop and simplify your network layout if there are other routers or RE in your network, just leave the modem and the C9 on, disconnect other devices and run the Speedtest app 3 times. Please ensure the 2.4GHz wireless channel width is set to 40MHz and 5GHz is 80MHz. Here is the guide to modify the wireless settings on the router:
https://www.tp-link.com/support/faq/2292/

5. Please disable QoS on the C9 (if there is one) or try to reset it to the factory defaults, then reconfigure it from scratch.

Additionally, please refer to the instruction below to narrow down the issue. 
https://www.tp-link.com/support/faq/2170/

May it help.