@PAvd 

Hi, 
You can try the following steps and see if they help.
  1. Open the Deco app, go to More > ECO Mode, and ensure that ECO Mode is disabled (ECO Mode can limit performance).
  2. Go to More > QoS, and ensure QoS is disabled. If you don't see an on/off switch for QoS, set it back to standard and remove all priority devices.
  3. Go to More > Advanced > NAT Forwarding, and disable SIP ALG and UPnP.

If the same phenomenon persists, please confirm the following information.
  1. Please describe your network diagram in detail. 
  2. When you test the Wi-Fi speed, please simplify your diagram temporarily, turn off the satellite Deco, and turn on the main Deco only. Then connect your wireless device to the main Deco's Wi-Fi, check the link speed, and go to www.speedtest.net to test the download and upload speeds. Please send us the link speed and a screenshot of the download and upload speeds.
      How to check the associated/link speed on a wireless client

Best Regards