I see this too.
I have a 1Gig symmetric fiber internet connection.
I am using SmartConnect on wifi.
The router says it only has 400Mbp up and down, but a Speed test directly from the modem shows 800ishMbp up and down. (speedtest.net)
with my old Netgear R7900P my wired gaming pc was showing 400Mbps, my phone was showing 200Mbps
With the XR500 I only get about ~80Mbps on each.
I've tried adjusting QOS to give a single device more (or ALL) the bandwidth, but the speed doesn't change significantly after applying the changes.
iPhone X, with 2% of bandwidth (interface shows 6Mbps) ... Speedtest shows 81.1Down/60.9 Up
iPhone X with 50% of bandwidth (interface shows 200Mbps) ... Speedtest shows 72Down/60.3 Up
Tried power cycling wifi on the iPhone; same results
Device Manager shows iPhone connected on 5Ghz.
Tried wifi on my 2017 MacBookPro 15" with High Sierra as well.
I can see I am connecting via 802.11ac, transmit rate up to 600Mbps from my Wifi details
Mac has clear line of sight to the router.
They are about 15' from each other. Open room, no obstructions.
Speedtest shows 96.1Down/34Up
The Network Monitor shows no other devices on the home network else using more than 1Mbps