@zerathul 

Wire a computer directly to the switch and test the speeds to see if there could be a 100 Mbps bottleneck happening. Either by the switch negotiating 100 Mbps or something with either port or the cable being used for the Deco.

Another thing to possibly try is to take the satellite Deco to the main Deco, connect an Ethernet cable between them, then wire a computer to the second port of the satellite unit to test the speeds on that setup. If those speeds are fine, something around the switch area may be causing the slowdown, meaning the switch or port or cable.