@Carl same here - investment in 6 decos m9 to assure top coverage ( which it has ) and constant speed in the house - speed at principal deco 100 mbps , all other deco ( though next placed in the next room to main deco and every next room / next wall an additional deco ) go 10-18 mbps ... please let me know a on side solution, or otherwise ( if it is a deco m9 problem ) change equipment.... 6 decos is to much an investment to solve this on my own...

@CE-DK-1974 Very practical....wouldn't it be much nicer if it worked as designed?

@CE-DK-1974 I've done this, but still get the low speeds for devices connected via wifi as per my previous post and other people's posts here. It's not a solution that works.

@SteveRPenny 

try going into more, bottom right,  press QoS, at the bottom Total Bandwidth, press edit, select  Set Manually,

change Download Bandwidth to.   1024

and Upload Bandwidth to.   1024 then press save at the top hand right corner.

I would then reboot your main router,  bt or virgin or whatever one you have. 

@SteveRPenny 

You will never see high speeds out to the internet with these devices. The technology just is not capable of 1Gbps. 

In a previous post i mentioned that all network traffic is routed to TPLink. This is how they off load features like QOS, anti-virus and parental controls. You are not unable to truly disable QOS so it is always on. Having traffic traverse TPLink's network adds to latency and consumes bandwidth. I suspect they're limited on their side. 
 

Try connecting any device directly to your providers modem you will significant performance improvements. Do a simple trace route and you will see your requests flow in a different direction and latency is significantly reduced. Do the same connected to a Deco and you will see the first few hops are hidden and have a high latency. I see a 30ms latency and bandwidth never exceeds 500Mbps, typically less. Direct connected, I can see bandwidth greater than 800Mbps and latency drops to less than 5ms. 
 

Internally I can consume 1Gbps moving data across my home network. Further proving the point that tplinks overhead is consuming a lot of bandwidth. 
 

Also, the speed test apps do not tell the truth. They are limited on their side, your device is consuming CPU and memory to complete the test which also takes away from cycles and power available to your network card. Simple HTTP also has limitations on how much it can handle. You cannot rely on these results. With that said I would recommend you use an app, not a URL/website to test your bandwidth. There is some more capabilities here and the numbers are more realistic. 
 

These devices using mesh are simply sharing a 1Gbps connection. You will never get a full 1Gbps throughout as other devices are set with some minor priority to reserve some of the bandwidth. There is no dedicated 1Gbps end to end, it's 1Gbps shared. 
 

Lastly, every device itself has limitations which will manage the bandwidth it can consume. My iPhone XR, sometimes connects at a slower speed rather than N. I'll never see the 300Mbps speeds N is capable of if the device isn't in the perfect location and connected to the N network. My windows 10 PC also cannot handle a full 1Gbps, even though it has thee network card to do so. The operating system and application slow down the processing, etc. 
 

I would suggest if you have good coverage around your home, devices stay connected and you can do what you need to do without issues this is as good as this device will ever be. 
 

There are newer technologies using more powerful network components, not sharing a single 1Gbps link. You will notice these are marketed to folks with internet connections over 1Gbps. Simply, the TPLink technology is older now. 

@Bbhui I don't have that option as I'm in Access Point mode.

@SteveRPenny 

I think a lot of the speed is device specific and proximity to the deco specific. Sitting in my home office with a deco (slave unit) hardwired to the primary unit my Linux laptop consistantly gets 145 up 170 down. My iPhone XR is seeing 470 up 345 down. Both devices are next to each other and both are within 3 feet of the deco unit. My Windows 10 desktop hard wired into this deco unit gets 240 up 300 down. All tests performed using speedtest.net

If I download and test using the speedtest.net app on Windows 10 I get 450 up 410 down. No change on the iphone.

However, walking out of my office and anywhere on my second floor I drop over 100Mbps on the iPhone. The Linux laptop stays consistent. Moving to my main floor, the devices stay connected to the deco onthe 2nd floor in the office. The bandwidth drops signficantly to an averga ef 40Mbps up/down on both devices. 

Forcing a connection to the main deco, which is located in my basement number improve to an to about 1/2 of what I see in my office on my iphone, but the laptop is consistant. 

My kids Samsung tablets show much larger drops in bandwidth the further away you are from the unit. These devices will even lose connectivity in a couple of corners of my home. 

I do find that the wifi hand off between deco's isn't great. I find my devices will primarily stay connected to one unit, whichever one is closest when they enter the home or turn on. I can walk down to my basement from my office and my laptop and iphone will stay connected to teh office Deco even though the basement deco is much closer and visible. This could be part of your challenge too. 

I do have fast roaming diabled, as it did mess with some older devices. I have beamforming enabled as I find it provides better performance.