Similar question. I am getting full/expected speed (200Mbs+ Download) when mesh connects me to the main Deco P9 that is attached to router, but when I connect to a satellite unit it is much slower - half or less.  What is causing the slowdown? This is a screenshot of one satellite P9 with my phone 2 feet from it. How should I interpret the 'source of the signal'.... Is wifi being used for the backhaul instead of poweline? Is it using 2.4GHz on the backhaul? What does it mean that it is not at full wifi strength, is this the quality of the powerline? Thanks in advance for the help. 

@Ced2ric I have the same issue, did you find a way to fix it?

I believe the slave P9 uses an hierarchy of connection (ethernet > wifi> plc) to use, in my case I believe that it picks Wifi even if the connection is bad, rather than using plc... This is dumb...