@Nick04 

If Main Deco connects to router (not just modem), you can switch Deco mesh to Access Point mode to see if this will improve its performance: Switching a Deco to Access Point (AP) Mode

In case you find you don't like how Deco mesh runs in Access Point mode, you can undo that change easily and will get Deco mesh back to its default Router mode.

  @Alexandre. I did it and nothing changed. Which is the best function? Mesh or access point ?

  @Nick04 

If you do not need features specific to default Deco Router mode, Access Point mode is better.

Try another test: force Deco mesh Main Network to broadcast 5GHz signal only, and check speed again. Also make sure that device you use to run speed test is connected wirelessly to Main Deoc and not more than 2 meters (6 feet) away from it, with unobstructed view.

  @Alexandre. I tried this too and nothing changed so switched to default again. Also for moment i reached 260mbps and the next moment again 150... If I reset all units and set the network again would you think it could help? 

  @Nick04 

You could try that. I would also suggest to ensure that when you run speed tests, your device is connected to Main Deco.

Try this:

1. Delete all Deco units from Deco mesh;

2. Delete Deco network from Deco app;

3. Add one Deco to new Deco network and it will be Main Deco.

4. For Main Network Select SSID you never used before, to make sure device connects to Deco. Make sure it is not the same as ISP router SSID;

5. Configure Deco mesh (with one Deco unit) to run in Access Point mode;

6. Configure Main Network to broadcast on 5GHz only.

Connect device to Main Deco SSID and run speed tests on it.