/**
 * Change product images on a single product page for mobile - ussue "properly size images" page speed
 */

add_action(
    'init',
    function() {
        // If not mobile device do nothing.
        if ( ! wp_is_mobile() ) {
            return;
        }

        add_filter(
            'woocommerce_gallery_image_size',
            function ( $size ) {
                // Using the size check we apply the new thumbnail size to the main image only not to gallery thumbnails.
                return 'woocommerce_single' === $size ? 'woocommerce_thumbnail' : $size;
            }
        );
    }
);