sterretje:
Lastly, I suspect that you're very close to the limits of an AVR based Arduino (Uno, Mega etc) if you sue the analog inputs; one analogRead takes approximately 100 microseconds.
What's the analogRead (ADC) time for a DUE?
I'm guessing that since the DUE clock is 5 times faster than the UNO's, that the analog measurement time is 100/5 = 20 usec.