Why Your WordPress Site Is Slow on Mobile & How to Fix It

<?php
/**
 * Automated Script Deferral for Mobile INP Optimization
 * Defers non-essential JavaScript to free up the mobile browser main thread.
 *
 * Engineered by WPDepartment.com
 */

if ( ! defined( 'ABSPATH' ) ) {
    exit;
}

add_filter( 'script_loader_tag', 'wpd_enforce_mobile_script_deferral', 10, 3 );

function wpd_enforce_mobile_script_deferral( string $tag, string $handle, string $src ): string {
    // Never defer core jQuery or administrative scripts
    $critical_whitelist = [ 'jquery', 'jquery-core', 'wp-polyfill' ];

    if ( is_admin() || in_array( $handle, $critical_whitelist, true ) ) {
        return $tag;
    }

    // Skip if script already has defer or async attributes
    if ( strpos( $tag, 'defer' ) !== false || strpos( $tag, 'async' ) !== false ) {
        return $tag;
    }

    // Apply defer attribute cleanly to non-blocking assets
    return str_replace( ' src=', ' defer="defer" src=', $tag );
}