This is a weird issue. Have a tp-link deco m4 mesh system.  Added moca to provide ethernet backhaul.  I see on the app that ethernet backhaul is in use.  If I plug an ethernet cable into the deco unit, I get full internet speeds (500Mbps download).  However, if I connect to this deco via wifi, the download speed is very poor (inconsistent, but typically sub 20Mbps).  Note that I am right next to the unit when testing the wifi speeds, and get the same poor speeds on both my iphone and macbook.  The signal strength is high and i see on the app that I am connecting to the deco that I think I am conencted to.  
 

Help !