If you're really after fast readings, I would go with a multichannel external ADC such as MCP3208
Each conversion takes 3 SPI transfers, see page 21 of datasheet, at 2 MHz SPI speed,
so 12uS vs 110uS, and provides 12-bit results.
and will not have the annoying channel-to-channel 'interference't that onboard ADC suffers (and made worse by external analog muxes).
3 SPI pins, 4 chip select pins, 32 channels of readings.