So I manage a fairly large ios project and never really took the time to properly validate compile times, so I just started playing around with it and found some nice little surprises.

    let horizpadding = self.collectionView.frame.width - ((5*50)+(4*25))
    Expression took 1299ms to type-check (limit: 500ms)

Now obviously this expression could have been simplified long ago as a single constant, but the values themselves were written to help others understand what that constant meant (5 elements that are 50px wide) + (4 gutters that are 25px wide)