As discussed here, notifications/sms can be delayed up to almost 30 minutes because of how GCM handles the connection to the phone.
I managed to fix this issue by using an application which sends a heartbeat every 5 minutes, which helps maintaining the connexion between GCM and the devices. I'm no longer experiencing any delay in my notifications.
It would be great to be able to have the feature to send GCM heartbeat at specified interval right in the Join app.
For reference, HeartbeatFixerForGCM is the application I used, it's open source and available on Github.