By the time you get to post-production, it's hard to be pro-active in making significant improvements to a project that has fallen behind schedule. Remember that rolling snowball? It was small and easy to push around at the top of the hill, but now it's a huge heavy object, it's moving really fast, and you're tired from running alongside it all this time.
If you didn't do your planning right in pre-production, and if you didn't keep on track in production, you're going to have a hard time in post-production.