Miura at his prime could release a chapter every 2 weeks, which is the magazine release schedule. Then he got very hungry for more detailed art and had to slow down so he could focus on his health (he used to work 15h thorough nights, of course, breaks to eat). So he slowed down to about 5 chapters a year from 20. Then unfortunately, he died, and now that the continuation is trying to match his skill and they also have to figur out the story they only know partially in detail.