Suppose you spend 9 hours looking at the code and one hour actually editing code, and the latter is the only part that can be made faster (if we are to believe grandparent). Now, suppose this technology makes you 4x faster at editing code. Then you'll spend 15 minutes instead of one hour editing the code, and a total of 9 hours and 15 minutes. This is roughly 8% less time than you would have otherwise spent. Productivity, being the reciprocal of the time it takes for you to do the task in question, this is an increase in productivity of roughly 8%.