1) Divide the coins into three piles as equally as possible
2) Measure two of the piles that have the same number of coins (i.e., if all three have the same number, any two. If there is one left over, measure the two that don't get it. If there are two left over, measure the two that get them.)
3) If the two you measured are identical, the light coin is out in the non-measured group. Otherwise, it's in the lighter group.
4) Repeat the previous steps with the offending pile
Should be faster.
Don't know how to prove it's the fastest, though.