Why does jQuery has a "window=this" at the very begining and say it would speed up references to window?

When I open jQuery's source code I find this line.

var 
// Will speed up references to window, and allows munging its name.
window = this

Why and how this line will speed up?

From stackoverflow

• this would be faster for javascript to reference to, as compared to window which would have to be resolved to the window object.

  Crescent Fresh : That's just what the comment in the code states.
• 1. javascript functions have lexical scope
  2. jQuery wraps its entire implementation in an anonymous function
  3. when said function begins execution it is executing in the "global" scope (ie this == window).

  "window = this;" simply creates a local identifier in that scope so that references to it do not have to "bubble up" outside of the local scope to resolve.

  David : Thanks. I just noticed that the code's running in a function scope. (function(){var window=this;...})();