“How come all the American yeshivos learn so slow?…”
First of all, compliments to you for thinking and facing this important issue head on.
You are correct in what you say. However, to say that all American Yeshivos learn so slow is not right. The derech of Sephardim and Chassidim is not like that. And there are Litvishe Yeshivos that don’t do it either. You just have to go beyond the narrow groups that do go too slow and get an arrangement that allows you to learn faster. Later on in life, after you have learned much Torah, you will have time to go through it again at a much slower pace, if you wish. But first, when you are young, you must learn a large quantity of Torah, at a quicker pace.
On this inyan there is a great kuntrus called ??? ???? ?????, with haskomos from Rav Moshe Feinstein, Rav Yaakov Kamenetsky and Rav Schach, ??”?. Take a look at it. You can see it at http://www.hebrewbooks.org/22633