On the 80/20 principle
This is observable everywhere in life.
Therefore if you pay attention to your efforts and results, you can single out the critical minority of things having the greatest effect, and drop the time consuming, ineffectual majority of efforts giving you little joy.
Parkinson's Law is also worth bringing up here. Work expands to meet the time you have to do it. That's quite a double whammy. If you show up having decided preemptively to work eight hours or more, and get bogged down in the less effective inputs in whatever you are working on, it is easy to see how you can spend a lifetime doing not that much.
The inverse of this is powerful. If you constrain how long you have to do something (set ambitious deadlines and work in a focused manner within that timespan) and tend toward the more critical inputs, you'll do a lot in very little time.