Adding a third Airport Express causes dramatic network slow down When TWO AirPort Express are on everything works as expected, strong and fast wifi everywhere, music streams fine. But when a THIRD AirPort Express is on, the wifi slows down to about 100k/s

This is independently from which of the three AirPort Express is turned off (so not a malfunction with a particular one) I have run wireless diagnostics which finds no problem.

Can anyone help, this is driving me mad.

Background

I have one AirPort Extreme serving as base station and three airport express (all latest model) serving as relay stations and to stream music in different zones, they are all connected by ethernet, the ethernet network has been checked and works fine.

I followed the Apple instructions and they are all setup as "create wireless network" with the exact same network name, in the airport utility app the three airport express appear underneath the airport extreme, wireless settings are on "automatic"

There is no other wifi routers, modems or printers in the building, but we are in the very center of the city and there are lots of wifi activity around.
This sounds as a channel overlap problem to me. Apple Airport devices try to use distinct and far-apart channels but may make a wrong automatic choice when there is a lot of interference from other WiFi stations, sometimes depending on the order you turn them on.

If you really need four access points (Extreme + 3 x Express) you either live in a really big house or those APs are not optimally placed. I'd suggest first using one AP (the Extreme), than go around every room and plot your signal strength and take note of other networks that pop up with their channel info. That way you find dark spots in the house and strategically place the Expresses to cover them.

Additionally, if you only need the AE to Airplay music to the speakers on the audio port you actually don't need to create an additional wireless network.