Read-only objects don't cache the converted v8 values and reconvert every time a field is accessed. This causes the following two major issues:

1. this.x != this.x if x is an Object
2. Since arrays are not lazy we need to build the entire thing each time we access. This is ok if the array is assigned to a variable then accessed through it but causes O(n^2) performance when accessed through a readonly object.

Example:

function mapFast() { // O(n)
    var array = this.array;
    for (var i=0; i<array.length; i++)
        emit(null, array[i]);
}
function mapSlow() { // O(n^2)
    for (var i=0; i<this.array.length; i++)
        emit(null, this.array[i]);
}