Changing the reference voltage can't be done so quickly as switching between the analog inputs. This is due to the 100 nF capacitor connected to the AREF pin of the ATMEGA328 on the Arduino board. This capacitor needs time to charge and discharge. Figure 9 shows an oscilloscope plot of these settle delays. The time it takes to change the reference voltage from 1.1 V to 5 V is reasonably fast: 15 μs, but the change in the opposite direction from 5 V to 1.1 V takes at least 5 ms.