@fab1anch0 

This is not an expected outcome. In your network setup, you should be getting 300/300 on both Deco M5. Usually, in cases like that, the culprit is wired part of home network: cables, switches.

Are these two Deco M5 connected through the single Ethernet cable, or is there a switch between them?

Do you have laptop or desktop with gigabit Ethernet card to run speed tests when wired to Deco M5? If yes, I can suggest tests you can run, to narrow the cause of slow speed.

  @Alexandre. Both decos are connected to the same router (the ISP's) but not directly to each other. I will test using ethernet, but both decos used to provide 300/300 via wifi when measured at a close distance.

  @fab1anch0 

Sorry, I thought "both decos are connected with Ethernet cable" meant "both decos are connected to each other with Ethernet cable". 

What you have is much easier to test, and may not need laptop/desktop.

Deco M5 has "Test Internet Speed" in Deco app, under More. This reports speed of wired link between Main Deco and Internet. Run it. You should see 300/300 or close.

Then, follow this document to change which Deco is Main Deco, and promote your Satellite M5 to be new Main Deco: Changing the Main Unit for your Mesh Network. After that, run "Test Internet Speed" again. If you get 300/300 on your new Main Deco, that means both your Deco M5 have good wired connection.