What's more important in JS: performance, or the understandability of the code?

JavaScript
32

[quote=“felgall, post:31, topic:223000, full:true”]
Yes, this particular situation is one of the few where there isn’t a better way.[/quote]

Why are IIFE’s not a better way? Let’s compare the two:

Functions that reach out and change external variables.

var removeEvent;
var addEvent = function (...) {
    // using lazy evaluation
    ...
    addEvent = function (...) {
        ...
    };
    removeEvent = function (...) {
        ...
    };
    addEvent(...);
};

Compared with using IIFE’s to return the appropriate function:

var addEvent = (function () {
    // invoked immediately
    ...
    return function (...) {
        ...
    };
}());
var removeEvent = (function () {
    // invoked immediately
    ...
    return function (...) {
        ...
    };
}());

Are there reasons why IIFE’s are not a better technique to use for this type of situation?