Prototype plugin used for lazy-load LCP analysis in WordPress

wp-lazyload-lcp-fix.php

<?php
/**
 * Plugin initialization file
 *
 * @wordpress-plugin
 * Plugin Name: Lazyload LCP Fix
 * Plugin URI:  https://gist.github.com/felixarntz
 * Description: Omits the first content image from being lazy-loaded, to improve LCP performance.
 * Version:     1.0.0
 * Author:      Felix Arntz, Google
 * Author URI:  https://opensource.google.com
 * License:     GPL v2 (or later)
 */

function ___img_lazyload_fix( $skip_images = 1 ) {
	$first_post_img_count = 0;

	// Skip lazy-loading for the featured image of the first post in the current
	// main query loop, if is also displayed before any other images in the post.
	//
	// In WP core this should probably be achieved using a new separate context
	// instead, e.g. "featured_image".
	add_filter(
		'post_thumbnail_html',
		function( $image, $post_id ) use ( &$first_post_img_count, $skip_images ) {
			if ( ___img_lazyload_is_within_first_loop_posts( $post_id, $skip_images ) ) {
				if ( $first_post_img_count < $skip_images ) {
					$image = str_replace( ' loading="lazy"', '', $image );
				}
				$first_post_img_count++;
			}

			return $image;
		},
		10,
		2
	);

	// Skip lazy-loading attribute on the first image in the first post in the
	// current main query loop.
	add_filter(
		'wp_img_tag_add_loading_attr',
		function( $loading, $image, $context ) use ( &$first_post_img_count, $skip_images ) {
			if ( 'the_content' === $context && ___img_lazyload_is_within_first_loop_posts( 0, $skip_images ) ) {
				if ( $first_post_img_count < $skip_images ) {
					$loading = false;
				}
				$first_post_img_count++;
			}

			return $loading;
		},
		10,
		3
	);
}

function ___img_lazyload_is_within_first_loop_posts( $post_id = 0, $first_x_posts = 1 ) {
	global $wp_query;

	if ( ! is_main_query() || ! in_the_loop() ) {
		return false;
	}

	if ( ! $post_id ) {
		$post_id = get_the_ID();
	}

	for ( $i = 0; $i < $first_x_posts; $i++ ) {
		if ( isset( $wp_query->posts[ $i ] ) ) {
			$check_post = $wp_query->posts[ $i ];
			if ( $check_post instanceof WP_Post && (int) $check_post->ID === (int) $post_id ) {
				return true;
			}

			if ( is_scalar( $check_post ) && (int) $check_post === (int) $post_id ) {
				return true;
			}
		}
	}

	return false;
}

___img_lazyload_fix( 1 );

See this make.wordpress.org post.