DIAGNOSIS The wireless backhaul between units is congested or using a suboptimal band. The satellite can only provide speeds as fast as its link to the main unit.
FIX Test with a wired backhaul. Connect an Ethernet cable from a LAN port on the main Deco to the satellite Deco's WAN/LAN port. The system will automatically switch to using the cable. Re-test your speed.
IF THAT DIDN'T WORK Check for channel interference. In the Deco app, go to 'More' -> 'WiFi Settings' -> 'Operating Channel'. Set it to 'Auto' and run a 'Network Optimization'. This forces the system to scan and pick the cleanest channels.
HEADS UP The 6 GHz backhaul band is fast but has very short range and is easily blocked. If you cannot wire the units, you must place them much closer together, with no more than one wall between them for optimal performance.