[wellsjohnston]

This does not have to do with AWS Lambda, I'm thinking about renaming it to "functional-s3", or something similar.

To answer your question, there isn't really a workaround for this yet, although indexing should be much quicker than "days". All the keys are listed recursively before running the lambda expression locally. If you have a huge number of files, this can take several minutes, maybe hours depending on the scope.

A workaround I've been considering is using a generator function to list the keys; that way, the lambda expression can start immediately, generating keys as it needs them.