The backend has php scripts that either return json objects or simple responses
I am a PHP developer and I ran into this question recently. At the startup where I work, we have a web app that is completely Ajax driven. When we decided to convert those Ajax endpoints to an API to be used with a mobile app and possibly open it to the public, I did some research and eventually decided on node.js and Restify. One of the draws is that with an Ajax webapp, we already have a team of JS devs on staff, so there would be less of a learning curve. I'd suggest looking into it.
EDIT: I forgot to address the multithreading part. I believe that node.js is not true multithreading...it is event driven, so it can use a single thread more efficiently, which should achieve the same effect.
Thanks. I started to look into Pyjamas which is all goodness of GWT but written in Python. Node.js and Restify look good. I will need to explore more on these.
The main reason I wanted to have multithreading in my app was to send notification emails on a separate thread than the main one, when an action is taken by a particular user and others interested in that action and are subscribed for notifications. It's like follow/notify model. I haven't found a solution for this yet.