As of 1.7.4 (though I could be mistaken), active hoppers cause extreme lag (essentially freezing the game) when traveling a certain distance away. I believe this occurs when their chunks are unloaded.

I use hopper clocks in many of my worlds (see screenshot). All of these worlds became extremely laggy. The lag occurred only when I was a certain distance from the hoppers--about the right range to unload them. I destroyed one such clock, and the lag disappeared. I then rebuilt it: lag returned. I then turned the hoppers off with redstone, and the lag disappeared. I turned it back on, and the lag returned.

Based on these observations, I think that when an active hopper (ie, one passing or receiving inventory and not turned off with redstone current) becomes unloaded, it lags the game to the point of freezing. I don't know if it matters, but in all cases the hoppers were above 128 Y.