Using lazy function definition it is possible to optimize quite consistently the performances of the event.fix() method.
This is a first tentative rewrite, I tried to adhere as much as possible to the restriction I learned in the forum, I mean return a writable event object.
I have executed the jQuery automated test suite and it passes all the tests, it also passes the "fix.html" and "offset.html" tests I found there. I have not built specific test using Custom Events but should work too with no changes for plugins developers (hope so).
For standard events I believe I have done more tests, and I have stressed it enough with the test I had already post in the forum.
Here is the DIFF patch to apply to the "event.js" source file,
the patch may be used to directly patch a copy of "jquery-1.2.3.js" if somebody want to help with testing without downloading the build.