The current implementation / use case of Autocomplete is that user needs to at least click in the autocomplete field and press down arrow key to show the whole list of the result or key in characters to show the matched list.
Your business requirement, "to show the autocomplete results after 750ms on focus of autocomplete field though user has not keyed in characters", is quite different from the current expected behavior. Most other customers may not need this behavior. pega_ui_autocomplete.js is a Final rule and generic for all customers, the modification will affect other customers too.
For your own testing, you can do private edit on pega_ui_autocomplete.js file and modify the code to test it.