@Peli 

Thanks for the reply.

Yes after sometime the study room will switch the connection to Ethernet, it may take days for the switch to Ethernet backhaul to take place.  I am using a TP Link 8 ports switch. Have tried swapping the cable and ports on the switch, didn't help.

I speed test using mobile phone, Huawei P30 Pro, connected at 866 Mbps, WiFi speed still running at less than 10 Mbps.

I found out that when I disconnect the Ethernet Cable from Deco x20, my WiFi speed will be 100 Mbps on my Huawei.

Suspecting something wrong with my  Ethernet Cable that connects from the study room lan wall plug to Deco x20, I changed out to a new cat6 ugreen flat cable. The WiFi speed on Huawei mobile jump to 390 Mbps, but it was short lived.  I took the Ethernet Cable out again to shift the Deco x20 in position, and then plug in  the Ethernet Cable into X20. The WiFi speed drop back to 10 Mbps,  I changed cables multiple tikes, reboot X20, did not managed to bring the speed back to 390 Mbps.

Either my lan connection has a problem, or the X20 deco unit is unstable. I tried a Deco m5 unit in the study room with Ethernet plug in, I get less than 10 Mbps.

So I conclude the following:

1. It's a lan cable or wall socket issue 

2. It's a x20 firmware issue.

I shall plug a Asus router to my study room lan to try, if my Asus does not have low WiFi speed, then it's confirmed to be a x20 firmware issue.

Update: I swapped the TP Link X20 Deco with a Asus 87U router. I connected the Asus with the same lan cable, the Asus managed to hit 100Mbps consistently on Huawei P30 Pro, connected at 866 Mbps. Once I move the same lan cable to X20 Deco, the WiFi speed drop to 10Mbps, with Huawei P30 Pro connected to X20 Deco at 866 Mbps.

Therefore,  this seems to be a firmware issue with TP Link X20 Deco.

Hope that TP link engineer can look at my post and advice what other tests can be done to troubleshoot the problem.

@HML 

Sorry that I fail to reappear your problem in our environment

May you connect laptop to the low-speed deco with ethernet cable ,and test if the speed is that low as well,check if the link speed of lan is 1Gbps.

Additionally, does all decos besides main deco have low speed, is there any other problems like some deco blinking red.

Try test speed with all other traffic stopped.

If any deco besides main deco does not have the problem,try transpose each other.

Try check the linght of switch which indicates the ethernet link speed.

Try if reboot the main deco can help.

Thanks for your time.

@Peli 

Same issue here. I have 900mbps download and 500mnps upload plan. Unable to maximise the max speed. the mesh seamless switch is not really seamless from main deco to satellite wireless connection. Currently I have 2 units of deco x20. Using Iphone 12 pro max, Main deco speed to 450 to 540mbps and satellite wifi is 120 to 140mbps. What is the recommendation settings for x20?

@Peli Hi,

Have the same issues: low speeds and the same problems: the maximum I can measure with any equipment connected with ax standard are 250/300 Mbits, and I have 1G internet base. The Vodafone router for itself can give me between 600/700 Mbits. I observe that the channels are always the same for 2.4 and 5Ghz and the truth is that they do not do any auto-management as it would be supposed, besides that the neighborhood uses the same channels with their routers, which explains these speeds. I ask whether or not they are going to update the firmware so that we can manage the most suitable channels.

@racmartins  The deco app has a network optimisation option, I manage to connect to 450+ mbps with iPhone 12 wifi on a early morning, probably less wifi congestion while my neighbours are still sleeping.