Object property lookups aren't free. The 'global namespace chain' code has to lookup the util property in the Library object, then the add function in the util object.

It's not particularly expensive, but if you do it in a tight loop 100k times it'll look slow compared to something that doesn't need to do that work.

As others have said, the trick is simply to perform those lookups earlier and assign the resulting function/data to a variable.