Also, since many tasks are literally only done once, "task" is itself a variable, not a fixed data point. As for "time", that can be quite hard to measure, since unless it's a very menial task it'll span several hours, or days even, and it's quite elusive to total what really constitutes real time spent on a given task.

I believe productivity/efficiency in software engineering is something that is felt (i.e you can measure some of its effects but many are a matter of perception, not definitive metrics. like pain.) rather than measured (i.e you can synthesize a definitive metric from a set of known, well-defined data).