May I know have you enabled QoS function and high priority function on the Deco APP?

If yes, on the QoS, please choose standard, and please disable the high priority function for all the clients, and then you can check whether the wireless speed will be better.

If you still have the same issue, please help to confirm the following information.

1. When connect a PC to the main deco by a cable, what's the link speed and the download speed?

You can use www.speedtest.net to check the download speed, and refer to the link below to check the link speed.

https://www.tp-link.com/en/faq-2265.html

2. When you connect to the main deco's wireless network, what's the link speed and download speed?

3. Have you tried different devices but the same result?