I can partially understand overtime when productivity is high enough that any improvements during those 8h would be negligible, and parallelism would not be a g

I can partially understand overtime when productivity is high enough that any improvements during those 8h would be negligible, and parallelism would not be a good idea (Brook's Law). Before you ask your employees to do overtime you should find out what their typical day looks like. I have mostly worked for startups during my career, and all the wasted time and interruptions still blow my mind. To make an analogy, it's like releasing a game with average graphics that a powerful GPU struggles to render, and blaming GPUs for low FPS instead of your poor software. You should optimize your game before you ask players to overclock their cards.