I have 5 TP-link M9 Plus units around the house and setup a wifi mesh network with the main Deco unit acting as a router connected over ethernet to my ISP's cable modem. The units are communicating wirelessly.

Lately, I have performance issues from all (no exception) wifi clients. Downloading is much slower than it should be.  I should be having more than 400 Mbps downstream (I have a 500 Mbps connection), but I'm only getting about 170 Mbps. I've tried with a Windows 10 laptop, two Macbook Pro 2013 and 2019 and an iPhone. Different speed-test websites or apps, all report circa 170 Mbps on all devices, no matter to which unit the device is connected.

So I've disabled Fast Roaming and Beamforming.  I'm not using Ipv6, VLAN, IPTV or MAC Clone. Also no Port Forwarding, Address Reservations, DDNS or SIP ALG. Everything is pretty much default and operating mode is set to 'Router'. I checked the Firmware but all units have the latest version. I rebooted all m9 units and rebooted all the wifi clients. Nothing has helped. Max download speed remains about 170 Mbps. When I do a "Test Internet Speed" from the main M9 unit to the cable modem/internet (these two are naturally connected over Ethernet), the download speed is exactly what my ISP had promised: almost 500 Mbps.

When I connect my laptop to the main unit using Ethernet,  I still(!) only have around 170 Mbps, so it's not specifically a wifi issue. I also disabled 2.4Ghz so I'm sure that they all connect over 5Ghz but this didn't change anything either.

I'm out of ideas.