[jQuery] about "speed up references to window"

[Jinpu Hu]

I see "var window = this" from latest svn code

I use firebug debugger it.

info:"Exception: redeclaration of const window"

but in core.js

var
        // Will speed up references to window, and allows munging its name.
        window = this,
        // Will speed up references to undefined, and allows munging its
name.
        undefined,
        // Map over jQuery in case of overwrite
        _jQuery = window.jQuery,
        // Map over the $ in case of overwrite
        _$ = window.$,

i want to know what does "speed up references to window"  mean?