DIAGNOSIS If your wireless speeds are fine but the wired connection from a Deco unit's Ethernet port to your PC is slow, the issue is almost certainly in that cable, port, or a software setting on the PC.
FIX Let's isolate the problem. 1) First, check the obvious: ensure the Ethernet cable is firmly clicked into both the Deco's port and your PC's port. 2) Try a different Ethernet cable if you have one. A damaged cable can negotiate a slow connection (like 100 Mbps instead of 1 Gbps). 3) Try plugging the PC into a different Ethernet port on the same Deco unit (the X20 has two). If speed returns, the first port might have a physical issue. 4) Reboot your PC. A stale network driver or setting can cause this.
IF THAT DIDN'T WORK Log into your PC and check its network adapter speed. In Windows, go to Network & Internet Settings > Status > View your network properties. Look for 'Link speed (Receive/Transmit)'. It should say 1000/1000 Mbps (1 Gbps). If it says 100/100 Mbps, the connection is being limited by the cable, port, or driver. Update your PC's network adapter drivers from the manufacturer's website.
HEADS UP The Deco X20's Ethernet ports are Gigabit. They won't be the bottleneck unless they're physically damaged. The culprit is almost always the cable or the device on the other end. Don't start changing deep Deco settings until you've ruled out the simple physical connections.