As someone who went from working a 5-day week to a 4-day week, I feel like a 4-day week is a 'sweet spot' - I'm just as productive as I was working a 5-day week, but when I work any less than 30 hours a week (e.g. when I take a day off), my productivity always goes down.
Working four day weeks, every week I'd think "damn, is it the Thursday already?" and on Sunday I'd be looking forward to going to work on Monday
Perhaps - but I suspect it depends on the person and the details of both job and workplace.
I spent about 6 years working a 3-day, and it went really well. However, it was more vulnerable to constant-cost outside factors like "unneeded meetings".
Let me support with an anecdote!
It seems much more likely to me that the act of changing it has a positive affect on productivity - but this is obviously a more short term gain.
Based on the reported outcomes it seems like it may have tapped into some discretionary effort, but doing that over a short period isn't especially remarkable, and I'd say there are easier and more sustainable ways to achieve that.
All the quantitative measurements given in the article are about the employees' happiness. Of course they felt they had a better work/life balance! Of course they felt less stressed! Of course they said they had more time to spend with their family!
They'd probably feel even better getting paid for five days and working three - and why stop there? Why not two, or one, or zero?
I don't doubt that most people in white collar jobs can be just as productive in four days, with the right incentives, as they currently are in five. But it would be nice, if only to justify the headline, to see metrics from the company's side rather than the parenthetical mention of "employees performing better in their jobs". Did they set up more trusts? Did they draft more legally rigorous wills? Did their customers report higher satisfaction and bring more repeat or referral business? If the "studies" aren't complete nonsense, the numbers were measured for these things. But then why aren't they reported with the same breathlessness as "overall life satisfaction increased by 5%"?