@garineuday 

Hello, please help verify some details, and we will try to fix it together:
1. Without any other devices connected when the speed on the computer drops significantly, please connect one computer to the C6 directly and go to speedtest to run the test for 3 times and show us the screenshots of the test results.
And please tell me the wired link speed at that time, how to get that? You can refer to the below link.
https://www.tp-link.com/faq-2265.html
2. What is the speed tested on the Tether app? It might be different from that on your computer, we may think the Tether speed is more like the connection speed between the router and your internet, which would be different from the speed you can get on the computer, while we can verify it first.
3. Do you enable any QoS setting on the router? Kindly show me the QoS setting page on your C6, try to disable it then check again.

4. If the wired connection speed from the C6 is good, try a mobile app to analyze the surrounding Wi-Fi, review all the channels in 2.4GHz and 5GHz. Next, review the channels on the router for 2.4GHz and 5GHz. If the channels match that around you, an indication of overcrowded. Manually adjust the channels for 2.4GHz and 5GHz to lesser use channels.

There are possibly other wireless parameters that need tweaking, such as Power Transmit, narrow or wide band, etc. but start with the above and below link:

https://community.tp-link.com/en/home/forum/topic/200588