I have flexible work hours due to being self hired, but once I get a commission, farewell to Cross Effects, farewell to BL, goodnight sweet Crusader Kings 2, I'm doing my job swift and efficiently because my client trusts me (I got even positive recommendations from my former employer. I worked for shale oil industry lobbyists and they even let me write a letter to American ambassador) even if it means I have to write stuff for 24 hours straight (my record).
It depends on the nature of your job. Personally, I simply cannot focus on doing something for 8 hours straight, unless it is
extremely interesting. All that ends up happening is that I get nothing done. But, at the same time, my job doesn't
require me to do so. For one thing, I often have to wait for programs to compile, run etc., and I also don't really have any fixed time when I need to work, so my work is also flexible. It's not a lack of effort, it's simply that I work in a different way. I have at times done work at home when there was something I needed to run.
Ultimately, I do not have the same view of work as you do. I will try to make sure I get everything I need to get done done, but I am not going to rigidly adhere to an 9-to-5 schedule. You do not have to be self-employed to work flexibly.