AirPort Express: Extremely high latency

I just installed a new AirPort Express today, and connected a MacBook Air and an HP EliteBook to it.

While browsing the web feels, to use an unscientific term, sluggish, I could verify serious latencies when pinging the computer from each other.

While sometimes the latency is as low as 2ms, most times it is around 300ms, spiking randomly even higher, and timing out from time to time.

This article suggest switching to 802.11n exclusively and setting the Multicast Rate to 2 Mbps.

While I tried changing the Radio Mode with no real success, I don't have an option to the the Multicast Rate on my device (Firmware 7.6.1).

When did this option disappear (or has it never been included in European models), and what other settings could help get the latency down?

Update: Using the trick from this question I got the response times down, and will now test how it helps with real usage:

sudo ping -i .2 192.168.1.1

Update 2: Even using the ping trick, I sometimes get timeouts from the the Windows machine and single digit packet losses. Sadly the performance is still very inconsistent, and I couldn't yet nail the cause down.

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