@CKFam 

Google this, to find detailed discussion on that topic: Range extenders can cut bandwidth in half because they rely on a single wireless radio to both listen for and broadcast data. 

To increase speed for Satellite Deco, your opitons are to wire it with Ethernet cable, or to replace your M5's with M9s.

Deco M9 has dedicated WiFi backhaul. On mine, with 300Mbps Internet connection, I am getting 300Mbps on Satellite Deco. Note that to achieve that you'll need both Deco to be M9: Main and Satellite.

Also, before doing anything else, you could try to disable 2.4GHz on Main network and run Deco M5 with 5GHz only. That might help a bit.

@Alexandre. 

Thanks for the information. I have little knowledge on these so just want to make sure. I think I will continue to use the M5.