If we're going to talk "practically" here :) things like branch prediction will ensure that nothing like 2x the execution time will be spent on just the double conditional.

Meanwhile, what you do inside that loop becomes much more important to the time estimate than the conditional (.length is a fast single property lookup, whereas just accessing a value in the array (which is presumably the purpose of this method) will often trigger a bounds check on length, then an offset into a looked-up offset into memory, etc).

So it's not really worth doing for speed. All that said, I think yours reads better anyway.