Switching from a channel with a large input range to a channel with a small input range can greatly increase the settling time.
Suppose a 4 V signal is connected to channel 0 and a 1 mV signal is connected to channel 1. The input range for channel 0 is –10 V to 10 V and the input range of channel 1 is –1 V to 1 V.
When the multiplexer switches from channel 0 to channel 1, the input to the gain selector switches from 4 V to 1 mV. The approximately 4 V step from 4 V to 1 mV is 200% of the new full-scale range. For a 16-bit device to settle within 0.0015% (15 ppm or 1 LSB) of the ±1 V full-scale range on channel 1, the input circuitry must settle to within 0.00015% (1.5 ppm or 1/10 LSB) of the ±10 V range. Some devices can take many microseconds for the circuitry to settle this much.
To avoid this effect, arrange your channel scanning order so that transitions from large to small input ranges are infrequent.
In general, you do not need this extra settling time when the gain selector is switching from a small input range to a larger input range.