gistfile1.js

// use case sample

this.$(this.selectItemsTag).select2({
            placeholder:"Enter an items",
            multiple:true,
            minimumInputLength:1,
            maximumSelectionSize:200,
            initSelection : function (element, callback) {
                var data = [];
                var val = element.val() || "";
                _.each(val.split(","), function(item){
                    data.push({id: item, text:item, label: item});
                });
                callback(data);
            },
});


// code from select2

        // multi
        setVal: function (val) {
            var unique;
            if (this.select) {
                this.select.val(val);
            } else {
                unique = [];
                // filter out duplicates
                $(val).each(function () {
                    if (indexOf(this, unique) < 0) unique.push(this);
                });
                this.opts.element.val(unique.length === 0 ? "" : unique.join(this.opts.separator));
            }
        },

The performance issue on setVal function when you have large list of items. The de-duplicate process slow thing down.