How to implement tag-like autocomplete for large select choice lists?

Using the select widget with its choices defined in the custom schema becomes a performance issue for large lists of choices, as they are fully loaded when the dataset edit form is rendered. E.g., 2000 lines of geojson for the spatial snippet will significantly slow down the read view.

@wardi could you point me in the right direction: Is there an elegant way to provide an autocompleting box (like the tag snippet) for custom lists of label:value pairs?

Start by tracing how the select widget loads choices from a custom schema during dataset edit-form rendering, then compare it with the tag snippet's autocomplete behavior. The issue does not name files or tests; done would mean large label:value lists are searched interactively without fully loading all choices into the form.

Written by the indexing model from the issue text.