"Failed to scale" means that async event-based programming becomes harder too quickly as the program grows in size. Performance isn't the problem, the problem is the program complexity explodes too quickly.

Twisted is world class; the fact that it essentially failed to escape its niche is one of the bits of evidence I submit for saying this style doesn't have some immense reservoir of success left to tap. Twisted basically tapped it, in a nicer language for what it is doing that Javascript is, and it's not enough to take the world by storm, in terms of solving the concurrency problem. Use it if it's the best solution, by all means, though. It's quality stuff.