@Aimannn 

Just got Deco M5. My internet subscription speed is 300Mbps down and 15Mbps up. I pluged the main M5 directly to modem.  However, when I tested with a Note 8 close to the main router, I only get 150/9. Even on a wired PC, I could not get 150/9. I prepared to return the Deco M5 set. 

One thing made me wondering is that if I test Internet speed with the Deco App, I could get 310/16. There must be something throttleing the speed on devices. I found I can set Bandwidth in QoS. So I set my bandwidth manually to 500Bbps down and 25Mbps up. Bingo, I can get 300/15 up now.

So, my conclusion is the QoS plays a load balancing role when testing speed on connected devices. The speed tested on router via QoS => Total Bandwidth is the real speed. 

@SXC I am having the same problem (with ethernet backhaul) and after 6 weeks of getting nowhere with TP-Link Technical Support, I am about to return the 6 M5 units that I purchased.

I had not touched the QoS setting since set up (nor set any High Priority devices) but Tech Support said to set QoS to standard. It required me to set a bandwith so I manually set it to 1gbps. It made no difference to my low wifi speed. I then set it automatically via intenet speed test and it told me the speed was 309mbps which is my contracted speed. However the Deco App FAQs say that the Deco app tests only the wired speed (Link #A) at the master unit (which is 300 mbps in my case) and is no use for testing the (Link #B) wifi speed received by devices around the house. As far as I can see, that speed test is irrelevant to this problem.

The Ookla speedtest is the one Deco recommend to test your device speed, and there is clearly some throttling happening if many people are getting the same issue. I have tested it in a 'clean' room with no other devices/no metal or other appliances that could interfere and using a new Macbook Pro with plenty of associated link speed. I have no neighbours within 100m. 

I am wondering whether these units are indeed speed restricted, contrary to their advertised specification, as I have same issue with all 6 units. If there is another solution, no one on this forum or technical support seems able to offer it, so I have lost confidence in TP-Link now.