@Holger 

This appears to be reporting bug in Deco speed test tool. Actually, it would be hard to get over 500Mbps from M5 on device connected to it by WiFi, but I have M9Plus in AP mode that also gets just 500Mbps in its speedtest, but over 600Mbps in speedtest from smartphone connected to it. Which would be impossible if Deco was truly throttled to just 500Mbps in AP mode.

Some people say that in Router mode this issue is not present and you'll see full gigabit in Deco speedtest, but I haven't tried to verify it.

Just ignore it. TP-Link is removing speedtest tool from newest Deco models, which tells me they would not put high priority on fixing that reporting bug.

@Swiftfire 

As I said, there is a bug in Deco speed test code, which manifests itself when Deco runs in Access Point mode.