Searches not Folders

If you file your emails into folders in your email program you’re wasting your time, according to a study by IBM Research. The 345-user study found that people who used the search function in their email program could find relevant emails as easily as those who had categorised each email into folders.

http://boxfreeit.com.au/Productivity/tip-want-to-be-more-productive-dont-file-your-email.html

The research found that those that searched found items faster than those that organized items into folders. Importantly, this did not include the time a user would spend organizing their folders - up to 20 minutes a day. The study also found that folder users treated the inbox as a to do list and folders gave those users a sense of task completion. Archive, archive, archive.

This is not so much a folders vs labels issue as how you use them. If you use Gmail's labels as a substitute for a folder then you will not get the efficiency of search.