connection speed problem with airport extreme

Hi, I´m using airport extreme and two airport express elements for best possible WLAN. It worked for a while, but from one day to another the connection speed from my apple device I wanted to connect became unrealisticcally slow. I have a 100 mb connection and the device, which are very close to the airports get below1 mb connection which make my work nearly impossible.

What is your idea, can you please help me ?

regards, jky