Kite, I have this information from the dozens of professional manga/comics artists I have met for years in festivals.
A page takes time: from the storyboard, to the sketching, inking and shading, it usually takes days.
Besides, there is a major difference between Japanese mangaka and European/American author: a Japanese author never
works alone, he has a full team of assistants in charge of backrgrounds, inking etc. That team can be composed of four to more than 12 assistants.
The work hours are different too, if you remember the week scheduleI had posted some weeks ago: Japanese mangaka have only three free hours each week and sleep less than 8 hours a day, with no day off.
Yes, they can make 18 pages a week, but remember they don't work in the same conditions as European or American artists...
and end up in hospital with huge hands problems or exhaustion symptoms more often than not.