I don't know if it's the fastest, but I've had good success using Puma with Ruby on Rails in a real-world production site. That said, if performance is your goa

I don't know if it's the fastest, but I've had good success using Puma with Ruby on Rails in a real-world production site. That said, if performance is your goal, then the web server is only a small part of it. In my experience, the way to get a high-performing Ruby on Rails application is primarily through fragment caching and using a CDN. You can get excellent performance with very little effort that way.