A new WordPress performance bottleneck is looming: admin-ajax.php.
More and more plugins are relying on the admin-ajax.php and the wp_ajax_ hook to deliver their content and functionality making WordPress more dynamic than ever. The problem is that if you have multiple plugins running all making their own wp_ajax calls without concern for what the others do, you end up with 4 or 5 server requests being required to render one page. This is a nightmare for server admins, especially since wp_ajax_ is often implemented as a POST request which allows it to bypass caching from cloudflare or varnish or what have you.
The solution is not to abandon wp_ajax_ but to come up with a better way to manage it. I really like what the Heartbeat API is doing. Instead of each client plugin managing it's own request, you can tie into an existing "heartbeat" to get the info you need. I would recommend all plugin and theme developers use the Heartbeat API approach if at all possible. At least until wp_ajax_ gets streamlined.