The last 20% takes time because you're in the realm of the last inch.
Now listen to the rule of the last inch. The realm of the last inch. The job is almost finished, the goal almost attained, everything possible seems to have been achieved, every difficulty overcome — and yet the quality is just not there. The work needs more finish, perhaps further research. In that moment of weariness and self-satisfaction, the temptation is greatest to give up, not to strive for the peak of quality. That’s the realm of the last inch — here, the work is very, very complex, but it’s also particularly valuable because it’s done with the most perfect means. The rule of the last inch is simply this — not to leave it undone. And not to put it off — because otherwise your mind loses touch with that realm. And not to mind how much time you spend on it, because the aim is not to finish the job quickly, but to reach perfection.
From Aleksandr Solzhenitsyn's book "In the First Circle".
I should add that as a pragmatist, sometimes you have to leave the last inch - reaching perfection isn't always compatible with life in the real world :/
