The op amp peripheral contains a voltage doubler for the analog multiplexer switches to ensure proper operation for a supply voltage below 2.5V. Apart from the multiplexers, no other supply voltages are affected by the voltage doubler.
The voltage doubler is normally switched on/off automatically, based on the supply level. If the supply voltage is guaranteed to be above 2.5V, the voltage doubler can be disabled by writing the Low-Power Mux bit in the Control Register (CTRLA.LPMUX).
When enabling op amps, additional start-up time is required for the voltage doubler to settle. Disabling the voltage doubler saves power and reduces the startup time.