See https://gist.github.com/kelunik/79f843c541aceb3927c5463cc643d502 for a reproduction script.

While the child process runs, php test.php consumes 100% CPU when running on the ExtEventLoop. Additionally, the connection is closed directly instead of one second later if there's currently a child process running.

See also https://twitter.com/kelunik/status/917757971790286848 and replies.