convection

Time taken to decrease the temperature of the whole room would be longer.
Since hot air is not as dense as cool air, so that hot air will stay near the top of the room. When the air-conditioner is installed near the floor, the cool air will stay near the floor and the warm air will stay near the top.
In order to cool the whole room, the cool air need to get to the top to cool down the warm air. So that it's not a very good idea to install an air-conditioner near the floor.