My first gig was at a start-up. They could only afford to pay someone for ~10 hours a week, which fit perfectly with my school schedule. When I left school, they offered me more hours, but I was adamant about working no more than 25 hours, and by that time they had seen how productive I was at just 10 hours a week. There were definitely some rough spots when they had tight deadlines, but for the most part, they scheduled things so they worked with my schedule.

My current gig I got through a personal connection. They were in desperate need of a senior level front-end dev, and I was just upfront with them from the start that I would only be interested in working 25 hours a week. It helps that they have a team of devs to rely on when I'm not there.

My advice: if you want to be part-time, either be an expert in your field, or find an early stage startup that can't afford a full-timer. This sounds kind of shitty, but it's hard to convince people who are ingrained in the 40 hour work week. Also, telling people you're a student helps. I've found that people look down upon me when I tell them I have no desire to work 40 hours, but if I tell them I'm a student, they're all for it.

One of my past employees worked with me for about 6 years, doing four days a week. He asked if it was an option and, while there were days when it was inconvenient given that I run a small business, I didn't mind. He's since moved elsewhere, but I would've kept him on doing three days or fewer hours spread over the week.

I wouldn't feel comfortable paying full rate for part time hours under the guise of it being "just as productive" though. I believe it can be just as productive but it would be too disruptive to face emergency situations or social situations that pop up during work hours.