Which framework/platform is a better choice for real time web application?

I'll start a new project in a few weeks, which in overall terms needs to update information in real time and have multiple users connected and receiving those updates, something like Google Docs where everyone sees in real time the changes in the document, and for example, having 20 documents open at the same time, and in each document there are 5 users receiving updates of the document.

I have to choose a framework/platform that allows me doing something like that but I need to have in mind that performance is a priority, because the system can increase the users online in a small period of time.

I been reading about what can I use to build something like that and it seems that the MEAN Stack and Ruby on Rails with Cramp let me do it, also I read about using Redis for RoR but it seems that using it like that can cost me a little bit of performance in the future.

My question is which one could be better to build something like this? given that concurrency and performance is a main concern, or maybe I'm overlooking another framework/platform that will allow me to achieve something like Google Docs.

Thanks beforehand for the help.