I observed the process hanging on for a very long time on several occasion, that I believe were related with pushing to https://web.push.apple.com/....
With debug turned on I could not reproduce the delayed response, but I noticed 403 Forbidden responses from Apple.
Is the bundle known to work with Apple ?
And to prevent hanging response, what about adding a timeout in sendAsync options at src/Sender/PushMessageSender.phpL93 ?