Test setting WebP quality

low-webp-quality.php

<?php

/**
 * Use the `wp_editor_set_quality` filter for testing.
 */
function set_webp_quality( $quality, $mime_type ) {
	error_log( sprintf( 'wp_editor_set_quality mime type: %s', $mime_type ) );
	if ( 'image/webp' === $mime_type ) {
		return 1;
	}
	return $quality;
}

add_filter( 'wp_editor_set_quality', 'set_webp_quality', 10, 2 );

/**
 * Specify WebP file output.
 */
add_filter( 'image_editor_output_format', function() {
	return array(
		'image/jpeg' => 'image/webp',
	);
});