@holsen1 I'll just add to the original message.   I took the main Deco Unit which was connected to the Router via CAT5 by about 60 feet of cable and went and directly connected it to the router over a 2 foot cable.  Rebooted it and got 780 Mbps wirelessly on my phone a couple of feet a way.   I brought that same Deco back to my office and reconnected it to the CAT 5 that runs from the modem back to my office and again got 770 wirelessly and 1 Gbps on my desktop hardwired to the Deco XE75.  I ran the speed test one more time and everything is choked back to 480 - this is driving me crazy.....  Why why why would this be?

  @holsen1

Hi David, thanks for chiming in with some assistance.

UGHH....

So here's what I did.   The Deco that was serving as main Unit was in my office and the unit in the living room (next to router actually) was simply a satellite.  I've got internal wiring in the home so I had the the Main Unit in my office via ethernet to the router.    So I simply reversed the two units.   The main router I moved from my office and placed it next to the router and the satellite in the living room I moved to my office and connected via hardwire to main Deco.   

After swapping the location of the two units - I ran a speedtest on my PC ha to the satellite in my office and got 980 Gbps.   I then check the PC in the living room connected to the Main Deco and got 780.  OK, not a Gbps but better than 400....

I ran another speed test and now the entire network is baxk to around 480 on PCs hardwired to the Decos while the confirmed speed from the ISP router direct is 1Gbps

This just makes no sense to me.

Just a little more Info - I notice that the the 4th Deco unit is Hardware V2 on Firmware 1.3.0 Build 20250722 Rel. 38703  While the other units are Hardware V1 on Firmware 1.4.1
I don't know if that makes any difference.  

I also wonder is severing the wired connection from my Office Deco and letting it just connect wirelessly to the main Deco would make a difference?

Ideas?