@Sospedra Is your ISP Modem in Bridge mode?

  @blkjak I'll check when I'm back at home. But if my Deco is outputing the correct mbps through Ethernet I suspect this is correct. It's just the WiFi signal.

  @David-TP Hi, thanks for the reply.

The Link speed is around 600mbps.

I have no QOS enabled.

I tried adding my device.

Then, I also tried with Deco transmitting only through 5GHz WiFi.

Nothing changed the result. My WiFi connected devices are capped at ~200mbps from the main unit and ~100mbps from the slave.

As you pointed, these models should output a bandwidth between 350-600 mbps. I don't see why should I buy a new TP-Link product when this one isn't performing as advertised. I don't intent to be rude here, just pointing that this should not be a problem fixed by throwing another ~300 euros.

Is there anything else I can try?

  @Sospedra 

I would suggest to focus on Main Deco and run speed test for Main Deco only. With the following:

1. Configure Deco mesh to run in Access Point mode. This will turn off all Deco traffic throttling features, such as antivirus, priorities, etc. How to set up Deco to work in Access Point mode

2. Configure Main Deco to only broadcast 5GHz:

3. Power off Satellite Deco. Leave only Main Deco running.

4. Position mobile device you run speed test on close enough to Main Deco so that it will report 866Mbps link speed. You may need to place that device 3-6 feet (1-2 meters) from Main Deco, with unobstructed view to it.

5. Make sure your mobile device does not connect through VPN. Having VPN enabled is definitely going to degrade speed test results.

6. Run multiple speed tests in that setup and share results.