Connection from mobile device to EC2 instance freezes

I deployed my app to EC2 today, using mupx. This was an awesome experience; I would have never expected that deploying would be that easy. Mupx is just a great piece of software!

Now to my problem (which does not seem to be related to mupx, but maybe to EC2?):

On a desktop browser, the app works as a charm, fast and reliable. I am still using a free-tier instance for testing purposes, but the response time is just great (ok, it’s just me using it at the moment).

But on my mobile devices (both Android and iOS), the app tends to freeze after a while. By freeze I mean that the server seems to take ~10-20s to send published data to the client. Sometimes, it is super-fast, but then again it freezes to this inacceptable delay. It could also be the client not sending the subscription, I just don’t know.

On the browser console, I cannot see any messages. I only had 1 message once, that said:

WebSocket connection to ‘ws://’ failed: Failed to send WebSocket frame.

But this only happened once.

Did anybody notice these kind of delays before and have an idea what I might have to change?

EDIT: I have not tracked the server using Kadira. It reports pub/sub response times of about 10ms (with 1 peak at 38ms). Method reponse times are even lower.