I just switched from a Netgear Nighthawk router to a Deco X20 mesh system with 3 nodes. I have a big house and wanted to expand coverage. I have 400 Mbps download Internet service. With the Netgear router, I was routinely able to get 400+ Mbps on Ookla Speedtest using my Galaxy S10. Now that I've switched over to the Deco X20s, the best I can muster on my phone is around 200 Mbps.

This is despite the 3 X20s using wired backhaul through a Gigabit Ethernet switch (unmanaged). My setup is Modem -> Main Deco -> GigE switch -> other 2 Decos.

Every room in the house is wired for Ethernet and all are connected through the switch, although the ONLY thing plugged into any of the jacks are the Decos.

I also have a PC with a GigE network card, which is wired into the second Ethernet port on one of the satellite Decos. On this PC, I am consistently able to get 400+ Mbps on Ookla Speedtest, which indicates that the main Deco is having no trouble getting full speed through the modem. That is, at least insofar as acting as a router/switch and passing on data from the modem to the PC.

Running the speedtest under the "More" page in the Deco app, I only get 200 Mbps. From what I've read, this is testing the speed at the main Deco. I have no idea why the main Deco would report 200 Mbps, and yet my PC can get the full 400 Mbps.

I have tried all of the troubleshooting steps recommended on the TP-Link support page regarding slow Deco WiFi speeds, including removing the QoS settings (I had none to begin with), setting the bandwidth to 1000 Mbps down / 1000 Mbps up on the QoS page, turning off UPNP, downloading a WiFi analyzer to verify there isn't a channel interference issue (there isn't). I've also tried unplugging the power cords from the 2 satellite Decos, and then running the speed test on my phone while sitting 10 feet away from the main Deco. I still only get 200 Mbps in that case.

I do have a lot of devices connected, but A) they're the same exact devices I had connected to the old Netgear router, and B) it's currently midnight - I'm the only one awake, and every device except for the computer I'm typing this on is currently idle. I have considered the possibility that there's an app running on one of my devices that is hogging bandwidth, but if so, it's now showing up in the Deco app, where you can see how much traffic each Deco is currently handling. All 3 Decos are currently reporting traffic rates in the Kbps range.

Any ideas what might be causing this, or how to go about troubleshooting at this point? I suppose a drastic next step would be to unplug the satellite Decos, physically power off every other device except my cellphone, and try the speedtest again. Btw, I'm not expecting to consistently get the full 400 Mbps on my cellphone during the day when everyone's videoconferencing, watching TV, playing games, etc. But I do expect to be able to get the full bandwidth that I'm paying my ISP for at midnight, with all devices except my cellphone sitting idle. Like I said, the Netgear router had absolutely no trouble providing that. I only bought the Decos to expand my WiFi range, but it's not worth it if it means throwing away half of my bandwidth.