@Vincern 

One would rarely see perfect results when connected to Satellite Deco. It should be expected to see at least some bandwidth loss when using range extenders, and Deco behaves as range extender. 

For clarity:

1) When running "Test Internet Speed" from Deco app, what download and upload speeds you are getting?

2) What is speed test result when mobile device is connected by WiFi to Main Deco? 

Also, what is signal quality between Satellite and Main Deco? If you get less than perfect signal, that would explain slower network speed.

You can see it in Deco app, here is an example of what I have with Satellite Deco M9 connected to Main Deco M9 (named Living Room):

@Alexandre. 

1. When I test the network speed. I am able to get only 50mbps. Lost almost half of the speed.

2. When connected to the main deco. I am able to get 90mbps. 

My signal bar from master deco was only 2 bars

@Vincern 

You might get better speed if you move Satellite Deco E4 to location where it gets stronger signal from Main Deco.

Before you do that, this is the test you can run to see network speed your Satellite Deco is getting from Main Deco now. Obviously, anything connected to Satellite E4 won't be getting faster speed than Satellite E4 gets from Main E4.

The test:

1. Power off Satellite Deco.

2. Place device you use to run speed test (smartphone, tablet, laptop) on top of powered off E4. That device will be connected to Main Deco during this test.

3. Run speed test from your device.

You can use the same approach to find better place for Satellite E4: while it is powered off, walk to where else you can move it, and run speed test from there with device connected by WiFi to Main Deco. The faster speed - the better place, of course.