Better to run air conditioner fan on high or low speed?
In very simple terms you are trying to "Condition the Air" not just cool it. There is a term "relative humidity" which affects the way you feel at certain temperature. Weathermen refer to it as a heat index. The higher the humidity the more uncomfortable you feel at a lower temperature.  So if you are running a fan at low speed it gives the compressor and cooling coil time to dehumidify the air rather than just cool it. On high temperature low humidity days running the fan at a higher speed will allow the air to cool down faster and reduces the air conditioner's ability to dehumidify.