Whether in terms of learning Gemara Tosphot and Maharsha or whether in terms of Physics, I think that there is no question that both "Bekius" (fast learning) and "Iyun"(in depth) are needed.

Clearly Rav Nahman was on the side of going fast. Saying the words just once and in order and going on as fast as possible.  And clearly at Shar Yashuv [Rav Freifeld's in NY] and in all places founded on the Gra, the emphasis is on the in depth approach.

But to me it seems clear that the Mir in NY settled on the path of balance. The morning was devoted to "iyun". There that meant preparing for the class of the four roshei yeshiva. And their classes were in fact amazingly deep. The classes were much more than going over Rav Haim of Brisk or R. Akiva Eigger. For some reason the roshie yeshiva had a new idea to add every day. [It was akin to taking classes in Physics from Richard Feynmann or Einstein.] I assume that this was because they had spent all their lives up until about the age of 50 simply preparing. So by the time they became teachers they had plenty of new idea every single day to go into the depths of Rav Haim Soloveitchik. 

The afternoons and evenings were for fast learning.