After the installation of the X20, the second pc on the second floor was painfully slow, getting just over 9 on speedtest and 10.0 Mbps on ethernet status. A day later, it improved to 88 and 100.0 respectively, well before my old router.
One support staff suspected that my network card needed to be upgraded. But my card could support up to 1000 Mbps. Anway, I opened up Device Manager to update the driver of the Realtek family card, but it said it was up-to-date. Then I followed the cue to update via Windows Update.
Eureka! After the updating, my ethernet status read 1.0 Gbps and speedtest 350 Mbps.
Support, please take note to update the knowledge base.