Backbone.NestedModel is a plugin to make Backbone.js keep track of nested attributes. It checks nested changes on .set() method call, which is also called internally when constructing Backbone.NestedModel or Backbone.Model.
It can easily be a problem if you initialize a Backbone.NestedModel with thousands of attributes nested in it, as it will take a long time for Backbone.NestedModel to find out what's changed, though this info is never needed when you initialize a model for the first time.
So next time you initialize a Backbone.NestedModel with complex attributes hierarchy, remember to pass the option { silent: true } like this:
var myNestedModel = new Backbone.NestedModel(myNestedAttrs, { silent: true });