It really depends on your environment and the complexity of the rendering tied to your collection. For example, on a fast desktop-class system with a collection that renders one element with 1 property, you can probably have hundreds (or maybe even thousands) of items in the collection before you start to notice slowdowns when the collections update. Working on lower-end and mobile platforms, those numbers change. Similarly, if you rendering is complex, it will take more time as well. All that said, there's really no "magic number"

There are definite workarounds such as setting up paging--but the simple approach of using Ember.js bindings to bind a collection to a template will fall to pieces as that collection grows.

I would also like to know this. :-)

me as well.