A longer slot gets you proportionately more time on the lesson material and can get you through the syllabus more efficiently with less total expenditure.
A European lesson is exactly that, by extending it you do not cover more of the syllabus, you simply spend more time on that lesson and you have the same number of lessons to do so it can't be more efficient. A student can only absorb about 40 minutes worth of teaching in one lesson so prolonged lessons achieve very little. Two a day with a reasonable break in between is about right. There is a saying, fast to learn, fast to forget!