Pre-packaged libraries are generally optimized for being composable and generic. When trying to get more performance, it can make sense to e.g. create an index

Pre-packaged libraries are generally optimized for being composable and generic. When trying to get more performance, it can make sense to e.g. create an index or lookup that can be used in multiple steps, or fold multiple iterative steps into a single iteration, or take advantage of the data's distribution to use a more specific approach, etc. You don't have to walk far from the beaten path at all before custom data structures start making lots of sense. Most interesting data structures in the literature are only implemented in a resuable, available way in one or two languages, if that.