AirPort Express not helping connection speed

Our office is on our main floor with a Time Capsule. On the same rack is the cable modem and a Drobo. In the office are two iMacs connected over WiFi.

At night we get terrible internet connection speeds using an iPhone from our bedroom (upstairs, opposite end of the house). That same iPhone walked into the office at night gets great speeds. So must be the distance of the Wifi. We got a AirPort Express, placed it in the bedroom and extended the network. This had absolutely no impact on our connection speed in that room.

What am I doing wrong?