Improve sampling speed while using a multiplexer
It's hard to offer much more than speculation without knowing more about the sensors and your current code, but here is one possibility:

If the bulk of your loop time is the sum of the conversion times, is it possible to cycle through the sensors twice, once to start each conversion (so they will convert nearly simultaneously), wait one conversion-time (or for the first sensor to complete), then cycle through them again to read back each converted value?

for each sensor,
  setup mux;
  send start conversion command;
end

for each sensor,
  setup mux;
  while conversion not done,
    ;  // wait
  read and store value;
end


For 16 sensors and assuming the conversion time the majority of the time spent, that could speed your sample rate to ~100Hz. Expect maybe half of that, since mux-setup and communication times are probably not negligible.