Comment by adrian_b

The previous poster was probably thinking about very low power analog circuits or extremely slow digital circuits (like those used in wrist watches), where the on-state of the MOS transistors is in the subthreshold conduction region (while the off state is the same off state as in any other CMOS circuits, ensuring a static power consumption determined only by leakage).

Such circuits are useful for something powered by a battery that must have a lifetime measured in years, but they cannot operate at high speeds.