@josemikkel 

As a first step, turn off 2.4GHz band in Deco Main Network, and re-run speed test on mobile device connected to Main Deco 5GHz.

If that does not make the difference, check this document for more suggestions: Troubleshooting: Slow Internet Speed with Deco Devices

@Alexandre. Thanks for this. We had already created a guest network that is purely 5G with a previous support team. 
 

Sadly we only get speed of up to 350mbps on main deco attached to the modem. I have another 5 deco units throughout the house and none of them get past 100mbps even if they show 2-3 bars signal strength.  

@josemikkel 

I have gigabit Internet, which allows me to test WiFi 5GHz performance to the max.

On Main Deco M5 I am getting up to 475Mbps. 

On Satellite Deco M5 the maximum I am getting is 225Mbps. This Deco is 20 feet and one wooden floor away from Main Deco. 2-3 bars signal strength.

I run Deco mesh on 5GHz exclusively and do not have much WiFi interference from neighbors. If I ever see someone's else WiFi 5GHz SSID in WiFi analyzer, it has very weak signal.

You didn't tell what is the speed of your Internet connection. I assumed it is 350Mbps. If it is, in fact, gigabit, perhaps you need different wireless device to run speed tests, because you should be getting similar speeds to mine from Main Deco. Satellites is different story, location and WiFi interference plays role, but not so much with Main Deco. Unless you swim in the sea of 5GHz WiFi networks with very strong signals, from your neighbors.

I run my speed tests on quite powerful Android smartphone, which is positioned by its manufacturer as high end gaming smartphone. No bottlenecks there.

Also, I use Ookla app for speed testing, which is recommended instead of speed test URL in browser.

@Alexandre. Thanks! I use Ookla as well and when on main router it is 350mbps but all other units only give mac 70-90. 
 

no wifi inteference as well as per the analysis tool