@Elmani 

Hello, sorry to respond late.

Please help verify more details and we will try to figure it out ASAP:

1. What is the current network topology/layout? How many Deco M5 do you have and how are they connected?

2. With your computer connected to the Virgin Media Hub directly, please verify the actual speed you can get via speedtest.net. Also, please confirm the wired link speed as the instruction below:

https://www.tp-link.com/support/faq/2265/

Please then connect your computer to the main deco and test the speed you can get from it via speedtest.net as well and check the wired link speed.
Note: Please show us a screenshot of the speed test result.

3. When connected to the main Deco wireless network, test this speed as well. Then test the speed by connecting to the satellite Deco and verify the speed. If the speed is good but still hangs when watching a video or playing games, please try to disable mesh for that device and confirm again.

Additionally, please refer to the below troubleshooting to check the QoS settings on the Deco app again:

https://www.tp-link.com/en/support/faq/2429/#case%201  (Case 1)

Good days.