The Io plasma torus exhibits a persistent lag from corotation with Jupiter, a phenomenon associated with mass loading due to local ionization of neutrals ejected from Io. The observed lag is the sum of two effects: slippage of the high-altitude neutral atmosphere relative to the planet and slippage of the torus relative to the neutrals. However, the relative weights of the two effects have not been well constrained. Recent observations show that the lag is very steady and insensitive to the position of Io. Models of torus chemistry suggest that mass loading should be concentrated in the vicinity of Io. If the lag is indeed a consequence of mass loading, then this inhomogeneity should presumably lead to more pronounced spatial and temporal variations than are observed. I show that because the neutral atmosphere has a much larger moment of inertia than the torus, it acts as a massive flywheel that requires several hundred hours to adjust to a changing source. By further implication, the effects of local time variations in the mass loading are also smoothed out, and the observed lag reflects the longitude-averaged source rate. This also confirms earlier predictions that most of the observed lag is accountable to slippage of the neutral atmosphere. Implications for radial transport are also discussed. ¿ American Geophysical Union 1995.