I just upgraded to 400mbps with Xfinity. I also got their modem with a build-in router. When I hook up my desktop computer directly to one of the two ports of that modem/router and do a speed test I get about 450mbps. However, the xfinity router misses some important features like VPN, IP address reservation based on MAC address, etc. Hence, I would like to use my Archer 3150. So what I did is disconnecting my desktop from the Xfinity router and plug that same cable into one of the LAN ports on the Archer 3150. Then I took a short CAT6 cable (CablesMatter, I think), plugged it into the port my desktop was plugged in before and then plugged the other side into the WAN port of the Archer 3150. However, now speedtests dropped to about 115mbps, i.e. about 1/4 of what I got when I connect my desktop directly to the Xfinity router. Why is that and how can I fix that?
Btw, I saw this other post that seems to have a similar problem where the Archer (different model than mine) speedtest gave wrong result and then due to QoS lowered the speed. However, I don't have the setting described in that thread and I only get 1/4 the speed compared to 1/2 in that post. I suspect these issues are not related, hence this new thread.