To add to Frumnotyeshivish’s points. Bekiyus if one does not know how to learn is of little value today (b/c seforim are widely available). The theory behind the modern Yeshiva system, as I understand it, is that we go slow in order to teach the Talmidim how to learn. As they get older they will be able to learn faster and faster until they can start covering a lot of ground while maintaining a high level of understanding.
In other words, the extreme iyun today is meant to be temporary. For example, many boys will learn very slowly in America and then go to the Mir in Yerushalayim and finish two large mesechtos a year.