I think part of the lesson here is that Disqus went very far with their realtime service using Python, a tool they were very familiar with and allowed them to iterate quickly. Migrating to Go took them very little time, once it became necessary.

I concur. I've been using go for close to a year now, discovered it inside Google and continued to play after leaving. It's really a fantastic language to work with. You can get away with using type inference most of the time which keeps the language feeling very dynamic. The inbuilt library support is very mature and provides almost all of what you need.

Hehe, I think he wants the traffic load problems more than he wants Go.

How would you normally go about stress-testing a Django app like that to look into bottlenecks?

All the articles and talks I see on scalability and optimization use production servers as examples, but as you way, you want to optimizing for traffic instead of responding to it.

There's no way to stress-test a complex web realtime system without actually running it on the real hardware.

You'd expect very nonlinear behavior between number of users, response time and number of machines used.

Not being a part of Disqus, the only way I see to "predict" performance beyond the load already seen is to try and use past data from the current real system.