Performance issues on search when node count is big

Problem/Motivation

When trying to implement a widget on an entity reference field that returns ~5k entities, site freezes when I try to enter a search query.

Steps to reproduce

- Install and enable the plugin the usual way
- Enable search on the widget
- Have a field of entity reference type that has a lot of entities as options
- Try to search

Proposed resolution

- Write an API controller callback function instead of current JS filter method in dropdowns class.
- Because the first proposition affects the performance on the server-side, figure out a solution to filter results efficiently with JS