Suppose you have something like

data Free f a = Pure a | Free (f (Free f a))
instance Functor f => Functor (Free f) where
  fmap = mapFree

-- Separated out for clarity
mapFree :: Functor f
  => (a -> b)
  -> Free f a -> Free f b
mapFree f (Pure a) = Pure (f a)
mapFree f (Free m) = Free (fmap (mapFree f) m)

Now void x = () <$ x, and `Free f` doesn't offer any potential for a cheap <$. So voiding out Free f a is going to cost you some. Laziness will help, but you'll face extra allocation.