I have worked part-time twice during my career. On neither occasion did it work out. The first time, I was a management consultant at McKinsey and, after my first child was born, returned there to work four days a week. Three months later my timesheets showed an average 55-hour working week.
As I was fully occupied with baby Tess on my day off, work really was crammed into four days and it was ugly - you can do the maths. McKinsey very decently backpaid me for the extra day, but this wasn't really the point.
I had also taken a project role that I was probably too comfortable with. This meant that while working furiously to deliver, I was, if not exactly bored, then certainly not challenged by what I was doing - a recipe for disaster. I left and went to work a full-time 55 hours a week in book publishing - as exciting as it was completely new.