admin-ajax.php is called with the following action: wpp_update. Look for whatever this action does, it may be doing a remote request, a big query, or something else that takes a long time to execute.
A quick search for wpp_update reveals the plugin to be WordPress Popular Posts.
This function is being called http://plugins.trac.wordpress.org/browser/wordpress-popular-posts/tags/2.2.1/wordpress-popular-posts.php#L360
There are quite a few queries there, but you’ll have to microtime() them yourself in order to find out how long the whole function takes to complete. The function appears to work for 1-4 seconds there but does not return any content; unless something depends on its “OK” response there (as far as I see, it doesn’t), it does not prevent the site from showing the other content immediately and should not raise any concerns whatsoever. It’s non-blocking.