Hello everyone,

I am a new TP-Link forum member and I would like to report a very unusual behavior that I have been able to reproduce consistently on my Deco X55 V3 system.

I believe I may have found a QoS bug or undocumented behavior on Deco X55 V3.

Device:

• Deco X55 EU V3.0
• Firmware 1.5.0 Build 20250619 Rel.69357
• ISP: Fiber connection, approximately 250/250 Mbps
• All Deco units connected via Ethernet backhaul

What I discovered:

1. QoS disabled
   Speed tests consistently show:

• Download: ~240 Mbps
• Upload: ~120-130 Mbps

2. QoS enabled (300/300 bandwidth configured)
   No priority devices configured.

Results remain almost identical:

• Download: ~240 Mbps
• Upload: ~120-130 Mbps

3. QoS enabled (300/300 bandwidth configured)
   One single Priority Device added.

Important:
The priority device does NOT need to generate any traffic.
I tested with an air conditioner device that was powered off and not actively using the network.

Immediately after adding that single Priority Device:

• Download: ~240 Mbps
• Upload: ~210-220 Mbps

4. Removing the Priority Device
   As soon as I remove the only Priority Device:

• Upload drops back to ~120-130 Mbps

Additional testing:

• Same behavior observed with Speedtest.net and Waveform Bufferbloat Test.
• Same behavior observed on both Wi-Fi and wired clients.
• Direct connection from laptop to ISP ONT gives approximately 240/180 Mbps.
• The specific device selected as Priority Device does not matter.
• The device does not need to be active.
• Simply having at least one device in the Priority Device list changes the upload performance for the entire network.

This makes me think that QoS is not fully activated when the Priority Device list is empty, even though QoS itself is enabled.

Can TP-Link confirm whether this is expected behavior or a firmware issue?

Has anyone else observed the same behavior on Deco X55 V3?

Thank you in advance for your time and assistance.