Yes, it seems that most people can't do more than about 6 hours of good work. And if it's something very intense, such as writing a difficult book, it's about 3-4 hours. The 8-hour working day was an arbitrary invention of industry, and it doesn't fit how humans naturally are.

When working from home, I tend to split my day into 1-hour blocks with 15-minute breaks in between. That works out at about 6 hours of actual work. But even then, I can be struggling a bit towards the end if I've been working on difficult stuff.

In an 8-hour day of office work, most people slack for about 2.5 hours of that time, checking social media, etc.