Fix sizes attribute to improve Lighthouse score for core/image and multiple core/images on the page.

functions.php

<?php
/**
 * Modified version of https://core.trac.wordpress.org/attachment/ticket/45985/45985-regular-images.patch patch for testing.
 * Full credit goes to that patch owner.
 */

/**
 * Add custom `sizes` attribute to responsive image functionality for post content images.
 *
 * @since Twenty Nineteen 1.0
 *
 * @param string $sizes A source size value for use in a 'sizes' attribute.
 *
 * @return string A source size value for use in a content image 'sizes' attribute.
 */
function twentynineteen_calculate_image_sizes_attr( $sizes ) {

	if ( is_admin() ) {
		return $sizes;
	}

	return '(min-width: 768px) calc(8 * (100vw / 12) - 28px), (min-width: 1168px) calc(6 * (100vw / 12) - 28px), calc(100vw - (2 * 1rem))';
}
add_filter( 'wp_calculate_image_sizes', 'twentynineteen_calculate_image_sizes_attr', 10 );