Could you please go into deeper detail on how these changes may improve the performance of ready()?

As previously written, ready() cannot be extended very well. My changes removed some arbitrary limitations to make this more realistic.

My specific use case was an implementation of $.include() that could be used in conjunction with AJAX. $.include() would include the files and wait for them to load. Meanwhile,

$.ready(function(){...})

could be used as normal, but would defer until the relevant includes were fully loaded. The basic idea was to make everything work consistently (especially ready()) even when content was loaded via AJAX, but without requiring all script files to be loaded at page load time. But I digress.

In doing the above, it became necessary to extend ready(). I know I'm not the first person to do this, (I believe previous versions of the live query plugin did this?) and I'm sure I won't be the last.

In version 1.4.2 ready() changed in ways that made it difficult/impossible/unsafe to reliably extend as needed. My patch includes 4 small tweaks that fix this. The code style is improved, and it becomes trivial to safely (and reliably) extend ready().

The changes and reasons are explained in comments in the code, but I'll cover them here too:

1) I added an _onReady function as a stub for ready(). This way addEventListener can bind to _onReady, and anyone overriding ready() doesn't also need to repair the listener binding. (At best, repairing the binding requires a complex and nasty kludge.)

2) Previously, the code used

readyList = null

to reset the list of ready functions. This is technically an error, and prevents the list from being reused if needed (such as when writing a robust include handler.) I changed this to reset using

readyList = []

instead, which keeps the variable contents consistent, and opens doors to future possibilities.

3) and 4) I changed the addEventListener events to bind to _onReady (instead of binding directly to ready, which is what they did before.) See 1 for an explanation of this.

These changes don't improve performance, they correct seemingly minor style issues that became a huge limitation later when writing a plugin. The edits are trivial and don't affect the normal operation of jQuery.

This was implemented in 1.4.3, not sure if it was based on this code or not.