I have noticed this too. I think a few things are involved. The first is the health of the plant---healthier plants move faster. The second is temperature---traps seem to close more rapidly on hot days than on cool days. The third is the age of the individual trap---young traps move faster than older traps.

I do not think that speed of trap closure correlates with the plant being "hungry." That would be cool if it were true, though.