You can really get a huge performance gain (+10x) with Swoole or any other event loop. The problem is that they require a different approach in your code and you cannot easily rely on 3rd party libraries anymore. You need to make sure that your app does not have memory leak, and can handle concurrent requests.
If I were to create a new app in PHP, I would definitely consider ReactPHP, Swoole, OpenSwoole, Roadrunner or the latest player FrankenPHP.