@Zetsu 

Hi, after the firmware update, please reboot all the Deco M4 nodes by powering them off then on to confirm if the speed will be back to normal.

If it is not, please follow the guide here to check the settings on the network, especially the QoS settings:
[Troubleshooting] Slow Internet Speed with Deco Devices

@Zetsu 

Hi,

Before the firmware upgrade, how much Wi-Fi speed you could get from the main Deco?

Would turning off the 2.4ghz on the Deco improve the speed?

Could you please help me run the following test so that we could forward your case to the senior engineers:

1. Power off the satellite Deco and only leave the main Deco;

2. turn off the 2.4ghz and leave the 5ghz only;

3. connect the mobile device and run the speed test—please send me a picture of the speed-test result;

4.check the link speed on this device- https://www.tp-link.com/support/faq/2265/  and send it to me(please also add the model number and operating system of this device)

Thank you and wait for your reply.

@Zetsu 

Hi,

I would like to forward your case to the senior engineers and please have a check of your email address later.

By the way, since the link speed on the Galaxy S10+ is 468m/s（For Deco M4, it is up to 866m/s）, could you please test one more device about the link speed as well as the final internet speed?

Sometimes the speed test server would also affect the speed, would it be possible to change a different server and test the speed again?

Thank you.

@Zetsu 

Thanks for the update.

Did you get an email from TP-Link?

I have forwarded your case to the senior engineers for further assistance and please check for the update via email.