I have 1Gbps up/down from my ISP. I get very nearly this throughput on my PC which is wired via CAT6 through two Archer A9 routers. The first of these is in my basement and connects to my fiber. The second connects to the first via CAT6 and is on my main floor. Both are configured with Wi-Fi so I can get it throughout the house.

For a long time I had both configured as routers, which meant setting up NAT forwarding for example was particularly complex as I had to forward through both of them. But Wi-Fi speeds were good (~500Mbps) throughout the house. Then I discovered how much simpler my network administration could be by switching my second router to Access Point mode. NAT forwarding is so easy! But...

I eventually figured out why every device throughout my house (unless it was in the basement, near my first router) was now maxing out at 30mbps. It's Access Point mode on the second router. I just reconfigured it as a Router and voila! I'm getting 500mbps on WiFi upstairs again. But alas, now my NAT configuration is harder.

Why does tp-link's Access Point mode make all Wi-Fi going through that device so slow?