Corollary to the "fifteen minutes" rule

I've read about some theory that says that it takes fifteen minutes before one starts working with full concentration. I can't actually find a good reference to this theory, but here's an article that mentions it.

I've been thinking about what that means for tasks that take less than fifteen minutes to perform. Or for writing that takes less than fifteen minutes to read. Or so on. If your brain is only half-engaged for these things, is there really much point doing them? Read a book instead of one hundred short news articles online. Devote serious time to problem solving rather than fixing niggles (until niggles are all that are left, of course). Write a book instead of a blog.

Because there's always something that will take more time than whatever you're currently doing.