Count me as a cynic if you must, but from 1962 until 1994, I managed employee groups ranging from a handful to 100 or so. Since, I've done a fair amount of consulting in the US, most of it in small operations (and admittedly mostly among the modestly compensated).
On the record, I'm comfortable maintaining that among the employees I've seen in action, the legendary 40 hour week had little relationship to actual performance or productivity. I've had employees who could in 10 hours do a week's expected work, and had others who took a month to actually work for 40 hours. The folks who worked hardest did so (in almost every case) because they wanted to, and matched with productive capacity, the results were grand.
Sadly, most of the folks who worked fro me either couldn't afford a 30 day vacation or already took 60 days off in bits and pieces, unofficially. For over half of the US's employed workers, "vacation" is no more than not coming in for two weeks, with activities ranging from steadier beer drinking to fishing near home to cleaning out the garage. If the European system was so much better than ours, their economies would be sounder than ours (which they're not). But then, we shouldn't struggle like the great mass of Heathen Chinee' or the majority of the employed and underemployed of the Indian subcontinent who can't afford to travel at all.
I've always suspected that the real decline (or the failure for demonstrable increase) in worker "productivity" in the US had more to do with the swelling ranks of the Byzantine bureaucracies of the "organs of the state", local, state and federal, where often constructive "output" can not be measured in economic gain. Of course, I'll admit that "privatization" doesn't seem to work any better.