I have a problem similar but not the same as this one:

Slow speed on DECO X60 - Home Network Community (tp-link.com)

I have an Xfinity 400Mbps download / 10Mbps upload plan.

The primary X60 is connected to an Arris SB8200 modem.

Connected with an Ethernet cable to the modem from a laptop (via a USB adapter) I get around 450Mbps download, 11Mbps upload.

Connected to Wifi from my phone or laptop I get around 85Mbps download, 5Mbps upload.

I, too, like the thread I linked, was able to observe for a short time, after the X60 main unit was rebooted, the expected speeds, and like that thread describes, the speeds dropped to under 90Mbps download.

I tested over wifi and over an Ethernet cable to the main X60 unit (same USB-->Ethernet setup I used to connect to the modem), and I got <90Mbps download and <6Mbps upload.

The difference from the thread I linked above is that the speed I see in the app on the phone, i.e. the internet speed as main X60 sees it, which is as expected, >400Mbps download and >10Mbps upload, so unlike the thread I linked, it cannot be the negotiated port speed, otherwise I wouldn't see anything above 100Mbps in the Deco app.

My network configuration: A main and guest network, both have 2.4GHz and 5GHz enabled.

About 35 devices connected to the main network (spread between the 3 mesh units), almost all are smart light bulbs, so not using a lot of bandwidth. I tried running the speed test over both networks as well as Ethernet connected to the main X60 unit.

What should I test or do next to resolve this?

Thanks for any attempt to help!