Answer by Aaron Batalion:
Having scaled multiple rails sites, a couple to millions and another to billions of PV/month, the runtime performance of the interpreter is never the slowest. Scaling a site is about architecture, databases, caching, event queues, disk IO, CDNs, etc. With a few exceptions, the runtime or framework doesn't crack the top 5.
Most important of all, Rails is the most efficient framework right now to build you app quickly and effectively. Stop worrying and start building.
Less than 1% of sites on the internet get to enough traffic to even care about performance…. but if you have performance problems, rejoice!
That means you built something people use!