skip-lazy-first-featured-image.php

// Add skip-lazy class ( or any class ) to featured image of latest post
function skip_lazy_class_first_featured_image($attr) {
    global $wp_query;
    if ( !is_single() && 0 == $wp_query->current_post ) {
        $attr['class'] .= ' skip-lazy';  
    }
    return $attr;
}
add_filter('wp_get_attachment_image_attributes', 'skip_lazy_class_first_featured_image' );