FieldWidget with autocomplete is really slow with massive referenceable entities

Problem/Motivation

The purpose of using the autocomplete feature is to speed up the rendering time of the select2 widget.
Since Select2EntityReferenceWidget extends core's OptionsSelectWidget, all possible values will be loaded on page load.

On a page with 10000 referenceable entities, this really slows down the page load.

Proposed resolution

Don't load all referenceable entities to initialize the widget, only the default values.