@TP-Link Thanks for the response. Issue has been resolved. Cause was the CAT 6 cable. Once replaced link speed is 1000/1000.

When connected via RJ 45 Cat6 1.5 metre cable to the spare port on the master Deco, my data speed as tested using Ookla is less than 1/3 of the speed indicated over WiFi. The Deco internal speed test...