I just bought the Deco M9 Plus system to get a better connection throughout my home. I have a 1000Mbps up and down connection from ATT Fiber. My internet gateway provided by ATT also has wifi capability that I can use for comparison. 

When connected via wifi to the ATT gateway, all devices are connected at acceptable speeds depending on how far away they are. My phone (iPhone) and laptop (Surface Pro) both connect at about 400mbps when they are within 4 ft of the gateway. 

My main deco router sites next to and is connected to this gateway via the provided wire. When connected to the Deco network, my phone tests at the same 400Mbps speed. My laptop though drops to 20MBps. I can connect the same laptop to the wifi network of the gateway, and it gets 400mbps again. 

So for some reason, Deco is limiting the speed of the Surface Pro. Other devices on Deco are getting appropriate speeds as expected. 

Here is what I have done so far: 

• updated drivers on the affected laptop. 
• Tried different MIMO SPMS settings. 
• Made sure no power settings are limiting wifi power. 
• QoS settings are set to standard/off
• I have tried turning off the malware/antivirus on the deco system, with no effect. 

Here is the funkiest thing of all: I have another laptop (dell), that was having the same issue. Turning off and on location service in windows 10 fixed the speed. The same would not work on the Surface Pro. I know some times wifi is used to improve location services, but not sure how this is affecting speed. 

On one side only the surface pro is having the issue, on the other, its only having the issue with Deco. It also seems like a limit rather than a performance issue. For example, for most devices, I get different speeds based on how far away from the router I am. But Surface is limited to 20 when its 4 feet away or across 2 rooms. 

Anyone have any ideas?