A motor nameplate is almost always a picture of the motor at full load. Only at full load will it develop the nameplate horsepower. Only at full load will it draw nameplate current. And only at full load will its shaft speed sag down to nameplate speed.
At less than full load, the speed will rise up toward sync speed until, at no load, it will be within about 1rpm of sync speed (due to windage losses, bearing drag, and seal drag).
In fact, assuming the supply voltage is correct, the shaft slip is probably the easiest and most reliable indicator of motor load available for field trades people.