that's a total misunderstanding of how the mining process works. every share you calculate takes (roughly) the same amount of time to calculate. your GPU's are churning through hashes as fast as they can and only submitting the ones that are "better" than the current difficulty. each one takes just as long as the one before and the one after to calculate, but only some of them are better than the current difficulty. it doesn't take any longer to calculate them - the difficulty just makes it less likely that any given calculated share will be good enough to submit.

the reason we are all so keen to have our GPU's have higher kHash rates is not because that makes them calculate a given share faster, it's because it measures how many shares they can spew out every second.