I worked in an excavating company for a bit. One old crochety guy worked 12 hours every day running an excavator. A younger guy who had stake in the company (also drove an excavator), who never worked more than 8 in a day, looked at him and said: “Why do you only get half as much done, but it takes you twice as long?”
The young guy wasn’t wrong. Being tired does slow you down. But yeah, a four day work week in construction, might slow the project down a bit. But they should just hire more people. And on top of that 6 hour days with additional staff would make the work go a lot faster.