Here's what I believe : the fraternizing and huddling around the break room during coffee breaks and distracting leisurely talks with coworkers at their desk, which all contribute to wasted time at the office would still happen in a 30 hour work week.

Except, Workers would be demanding overtime pay to finish the work they have normally been doing in a 40 hour work week and employers would have to pay premium compensation for the same level of work they used to be getting at 40 hours a week.