AirPort Express: Extremely high latency
The problem could be a huge amount of interferences produced by other Wi-Fi networks in the same band, 2.4Ghz band is shared by a lot of technologies (from DECT phones to Bluetooth and WiFi) and depending your area,neightbours and the way your building have been built, you can get a lot of issues like this.

In those situations i recommend switching to cable, it will always give you maximum performance if you are most of the time in the same place.

Another option will be to switch the band to 5GHz which is lot less crowded and much robust in terms of interferences, so if boh of your devices support that band give it a try! Your wifi will also reach some more distance