A bare minimum example of leveraging the Iron Ajax plugin. Recommended locations for this code in order by my personal preference: mu-plugins file, a plugin, or the theme functions.php.

iron-ajax-bare-minimum-implement.php

/* Modify the entries_count
 * i.e. the number of entries that need
 * to be processed
 */
add_filter(
	'fe_ajx_iron_ajax_entries_count',
	'examp_iajx_entries_count'
);
function examp_iajx_entries_count( $count ) {
	// generally you'll perform some lookup
	// to determine the total number of entries
	// $count = my_function_to_determine_count();
	return $count;
}

/* Process each entry
 */
add_filter(
	'fe_ajx_iron_ajax_process',
	'examp_iajx_process',
	10, 3
);
function examp_iajx_process( $response, $index, $persist ) {
	// add code here to process based on $index
	// e.g. process the row at $index in a csv
	// my_function_to_process_one_item( $index );
	return $response;
}