I solved the speed issue by placing the TP-Link Deco as an Access Point. It's useless as a router to manage wireless and wired speeds.

@David-TP Okay, so at my current home (not the new home where the Decos have now been relocated), I have installed my very, very OLD Linksys Velop Mesh router. Basically, I swapped out the Deco unit...

@David-TP Hi, I'm in the process of moving homes right now which explains my late reply. I have moved the three Decos to the new home and will test them once I am settled down. Your scenario is...

@Kevin_Z I checked it out. Nothing from the suggestion works. It's the firmware that's causing the slowdown. Didn't happen with older firmware. It doesn't properly negotiate a 1000 Mbps speed with...

My Deco's are capped at 100 Mbps when using wired ethernet. If I switch to wireless, it goes full speed. Big bug for TP Link hardware.

@David-TP thanks for response. Yes, the 2.4GHz won't see the 6GHz or 5GHz band, of course, but by enabling the same SSID name across all bands in Deco, it's causing instability with the IoT Wemo...