I have been using GTD for ~3 years and have reached a point where a paradox is clear. I hope you will comment on it and share your observations / solutions.
When processing, I take each piece at a time as GTD outlines. "What is it?" implies I read the item (e.g. an email) a first time. It can take a few minutes to understand the item well enough to define its correct next action(s) or project(s).
When doing, I must re-read the item to understand it well enough to do the action well. When reviewing, I re-read each again for project planning etc. Sure, I could write paragraphs to specify the next action in enough detail, but that would be worse still!
Given hundreds of emails each week, can the typical knowledge worker afford duplicating that couple minutes on each item once for processing, then again while doing? i.e. we're touching the item twice, not once.
I have measured that overhead for myself aggregates to an additional 6-8 hours per week, which I have been absorbing on weekends. Productivity methods & tools should enrich our lives, not drain one's personal time.
Further, when I have made the first investment of time to understand an item, my mind wants to pursue it. I force myself to abandon progress as a reinforcer of the GTD workflow in the hope it's a net-gain. After ~3 years, it still seems broken.
That is the essence of the paradox I face with GTD. My workload is not abnormal. How have you overcome this double-hit or lightened the overhead of GTD?