[pcwalton]

> pause linearly related to the size of new-but-not-so-young objects

Stop and copy still has to traverse all objects that contain pointers, even ones in the nursery (otherwise you'd incorrectly free objects in the nursery that are only referenced by other objects in the nursery). So the amount of scanning you do is proportional to all objects in the nursery (also the size of your stack/root set), and only the copy is linearly related to objects to be tenured.

> The second reason is that stack memory is usually a very small percentage of the total memory for interesting programs.

A small percentage of the long-lived memory, sure. But programs spend a lot of their time working on ephemeral data, and this is precisely why the generational hypothesis holds. There's no faster nursery than the stack: allocation fits nicely into the function prolog and deallocation requires no traversal of interior pointers.