@benbeara 

UK V1 

latest on website is 1.2.2 , update on app is 1.2.6

Download for Deco XE75 | TP-Link United Kingdom

I have come to the conclusion that firmware 1.2.6 introduce some slow down, stuttering on the network ( 3 XE75 - Wireless backhaul ) 25 devices. nothing scientific just using both firmwares for week or so and general responsivness on a 900/900 connection. 

Gone back to 1.2.2 via web downgrade using the bin file from the downloaded zip.  so waiting to see if a newer firmware is better ( not sure if any are planned for the XE75 ) 

  @IntrovertSins 

Major update - Resolution!

Firstly, I'd like to thank @IntrovertSins for telling me where I could find v1.2.2 as it's not available on the Canadian TP-Link website.  I downloaded it from the UK website, updated my router... and voila!  The lag is gone!  No muss or fuss with IPv6, and the "stutter" is practically non-existent.  This is true for all my devices, including my computers, my apps on my phone/tablet, and even the apps on my smart TV (e.g., YouTube, Prime, etc.).  I'm indebted to @IntrovertSins because before this solution, the situation had become untenable. So much so, as it started to have a major impact on my ability to work.  MS Team meetings would get messed up and disconnect me completely and/or garble my voice to the point that people couldn't understand me (i.e., connection instability causing audio stutter).

I was apprehensive this downgrade would work since the app prevented me from downgrading to v1.1.11.  However, that was not the case and now things seem to be quite stable for the past 6+ hours.

Thank you, thank you, thank you!!!

Update part 2

For clarity sake, I configured my network with wired back haul (1GB Ethernet switch) .  When I first set up my Deco units (Feb 2023), I noticed that the wired back haul wouldn't work and found out it was due to an incompatibility issue with D-Link switches.  So I upgraded to a TP-Link switch (Mar 2023) and it has worked flawlessly since then... until I upgraded to firmware v1.2.6.  I discovered this incompatibility only after doing much research on the net and various forums. 

Update part 3

Today, TP-Link responded to my support request and asked me several questions.  They wanted to make sure that my Internet service was stable so I sent them my speed test results with firmware v1.2.6.  The results were fine but it took a while to establish the connection.  After the download portion of the speed test, the "stutter" then reappeared and the upload portion of the test failed (three times in three attempts).  Their last request was for me to connect my computer directly to the modem and run speed test again.  This time, the test ran without any hiccups (whatsoever i.e., no lag, no stutter).  I tried the latter test (direct connection to the modem) on three different computers and they all worked without issue.  This results makes me believe that firmware v1.2.6 is truly the root cause of my problem as I'm not experiencing any issues with firmware v1.2.2.