@Wassbergh interesting these issues seem to be of concern again. I too am seeing lower performance recently, since the last code push. 2.4.15 is my current code version. 
 

im noticing my desktop which is wired at 1Gbps no longer has a speed test over 300Mbps up/down on a 1Gbps up/down fibre loop. 
 

Streaming to any wifi connected device is buffering and slower than it was previously. 
 

No change with QoS enabled or disabled. Doesn't make any difference if I use a custom QoS setting either. 

My personal opinion is that the software engineers and QA team aren missing something. It's like the code for the recent update was written on top of previous code, taking a step back in quality.  
 

I've also been complaining for a year or more now on the fact that these units choose the most congested wireless channels, not the least as advertised. Plus, there is no way to manually change the channel. Again, something I feel is wrong with the code. 
 

Getting TPLink to do anything or acknowledge an issue is likely never going to happen. I'm hearing newer devices from them also have issues. 
 

I personally will replace my tplink devices with a competitors product. Maybe Google? It's too bad, I like the size, but there's clearly a coding issue. They've got our money that's all they care!

@TWPS So... i was renting a G310 router from Fios and having issues with it (tried 3 different routers, hours on phone with Verizon tech support...).  Decided to buy my own device, hence the M9.  I have not yet returned the Fios router, so I did a bunch of tests after my post 2 days ago. On G3100 (no M9 involved) I could see 1G u/d... then plugged M9 into G3100 and suddenly the M9 started behaving at specs.  I plugged the ONT Cat6 back into the M9... and all problems went away!!!  Desktop (wired Cat5e/Cat8), which 2 days ago (before my post yesterday) wouldn't go any higher than 100Mbps u/d no matter what I tried... is now 700/500Mbps u/d.  My phone, from a remote unit (wifi backhaul to base), is also performing at 200+Mbps, as if it were Wifi onto the base (wouldn't go above 90 u/d from satellite unit 2 days ago).  Both of those 2 issues were fixed after my post. Bandwidth test done via Deco app are stable at 600/800Mbps d/u (despite me desktop seeing testing download speed at 700d ... werid).  I've not touched ANY configs in the app.  All i did was plug/unplug cables and re-route traffic.  QoS was 1024/1024 all along.

For the record, I tried to fix desktop stuck at 90 u/d for 2 weeks. I got Cat8 cable, PCI and PCIe 1 GB ethernet cards (thinking mine was broken), etc. and couldn't get it to speed up. Now it is 24 hours at roughly the performance level I was expecting when buying the M9.  For the moment, my big problem appears to have gone away.  I still think this spec listed on the box is a misrepresentation: "2 Gigabit ports WAN/LAN auto-sensing) per Deco Unit".  I've never seen anything above 700Mbps using known good cables (and network card) that saw 984 G u/d via the G3100 Fios Verizon router.

Anway, will post again if performance degrades.  Fingers crossed.  My code is 1.5.1 Build 20210126 Rel. 59387

@NotARobot 

I was looking to disable QoS earlier today on the deco app and I stumbled upon another post on the forum. I think we all know that you cannot disable QoS, all you can do is set it to standard mode, or swap the devices to AP which appears to disable all features. 
Here's what I learned. All traffic is sent through tplink servers as they control QOS and parental control. The devices do not have this intelligence internally, all "routing" features are managed by remote tplink servers. I'm sure other manufacturers follow a similar design, but maybe not?

This would certainly add to latency and slower throughput if all traffic is routed through tplink hosted management servers. 
The title for the other post is "Turn QOS Off - M9 Plus" sorry can't link to it currently. 
I stumbled upon this as I did a speed test with my provider today, for fun, which measures my internal network speed/latency from the device to the modem and in/out from public internet. It also tests to the carrier modem. I was shocked by the results. I'm barely seeing 100/150 d/u to a Windows 10 desktop with GigE connected through the tplink. Directly connected to carrier modem I see nearly full GigE throughout and latency drops from 20ms to 3ms. 
My solution is working for the most part, of course I'd like faster speeds to my devices. But this is crazy that all traffic is routed through tplink. 

@TWPS Wow!  Thanks for the info. Here's the link you referenced:https://community.tp-link.com/us/home/forum/topic/206632  This probably explains my speed fluctuations. TPLink routing all all my traffic... not surprised, was wondering how they cram all that functionality in the devices... not sure how I feel about it now that i know though.  Do I really need homecare and parental controls?  Verizon did the same, but they are US based and delivered 1GB do my door.  Maybe I'll swithc to AP mode and see.  I don't see any information anywhere on the product box/maual that indicates routing to their servers is necessary for HC and PC. smh 

@TWPS 

1 have the same issue. I got 3 M9 Plus (ver 2.0), all connected with other M5 and E4 through a gigabit switch. 1 M5 and 2 M9 works great over 100 mb, just 1 M9 it´s working on 100 mb. I can alson see the light in the switch indicating this. I try changing the location of the M9, inverting it with other M9 in an eth port that was working over 100mb, and happens the same. Firmware: 1.5.1 Build 20210126 Rel. 59387