Why does a PhD take a longer time in some countries?
Many factors that vary systematically between countries may affect time to graduation.

Here is an incomplete list:


In the U.S., grad school starts with studying for an MA degree; in Europe, an MA is a prerequisite to joining grad school
whether or not PhD studies include classes
availability, generosity and length of funding, vs. necessity to earn a living while working on the PhD
incentives to delay graduation vs. incentives and opportunities to join the labor market as soon as possible
quality of supervision and extent of leeway to pursue own interests and potential dead ends.


I'm not aware of any empirical studies testing each factor's relevance.