@Cronos 

Yes, Deco M5 can provide faster Internet, but there are limits of what it can do.

The best you should expect to get from Deco M5 wired to ISP router is about 500Mbit, when mobile device is using 5GHz. This is not the limitation of M5, but of WiFi5 protocol.

If Satellite Deco M5 is connected wirelessly to Main Deco, the download speed on devices connected to Satellite Deco will be less than that.

In any case, you should see much better than 100Mbit download speed after upgrade. I have Deco M5, it is wired, my ISP provides up to 350Mbit, I get 350Mbit download speed on smartphone connected by WiFi to M5. For your Deco M5 that are wired, you should get at least that, but probably more.

Of course, because Deco M5 has gigabit ports, if it is wired to ISP router anything wired to second Ethernet port of M5 should be running at gigabit, too.

I you want to utilize gigabit for WiFi connections, research mesh systems, from Deco or other brands, that support WiFi6. To utilize WiFi6 benefits, wireless devices must also support it. 

Congrats with upcoming fibre internet upgrade. You will enjoy faster speeds with Deco M5.

PS. If you run your Deco mesh in router  mode, after you've got fibre you may need to update QoS parameter in Deco app configuration settings, to unlock your new faster Internet speeds. See Using QoS feature of Deco.

@Alexandre. 

Thanks!