Thank you janui,
I've tried what you suggested.
Modem/router off. RuneAudioAP on, connected with my phone to the accesspoint and opened the RuneAudio app.
The same slow respons is happening (Well, maybe 15 folders instead of 10) . RuneAudio Raspi3B and my NAS are on a fixed IP.
It worked till the day before yesterday perfect.
Again, if I use Firefox on my phone, it works really smooth... (But I don't like Firefox)
What kind of information/diagnostics would be useful for you?
Jeroen.
Edit : I've removed all Chrome updates (back to factory default) but the problem stays the same.