Parkinson's law is the adage that "work expands so as to fill the time available for its completion".

I haven't had a full-time job, so take this with a grain of salt. But whenever I suddenly got more time to work on a particular project (e.g. because I hired someone to take work of my plate), it didn't increase my productivity as much as you'd expect. When we have little time to work on something, we end up spending it more wisely.

That doesn't directly address your question, but it's worth keeping in mind.