I switched to a 4 day work week (8h a day) with a 20% gross cut, which is about ~17% after tax, in February.
I had some wrist pain/discomfort (especially on Thursdays and Fridays) which was distracting for me. Since then it completely stopped. Way more effective than all other approached I tried (more exercise, more sleep, healthier eating, frequent breaks).
Since then I have had one evaluation meeting with my manager and we both agree that I have the same or even slightly better performance since. I have never felt that time was the limit of the quality/quantity of my deliverables.
Not worrying about pain, and most important not having the pain/discomfort is the main factor on the improved productivity. And to be honest, I feel like I got a raise, I have more free time for myself and spend less in health and in peripherals or gadgets to try to reduce my discomfort/pain.
And I have motivation to learn tech stuff and create projects of my own after hours and in the weekends, which I have slowly lost since it was physically painful to do so on a 5 day work week for me. This is important to me because I have always had the curiosity and interest to learn and create outside work.