That's like asking if a car goes fast enough to get you between two cities in one hour.
Depends on the car, depends on the distance between cities, depends on the driver.
Depends on the code and server, depends on the community requirements, depends on the programmer.
I'm not trying to be flip with you. You've just asked as question that's impossible for anyone on this list to answer.
Rails can be fast. Rails can be flexible. But it's not a magic pill. You will run into performance problems and pain points, and you'll have to work through them just like you would with any language or framework. Rails is also alive and changing (very much so). So "the performance" of Ruby on Rails is a constantly moving benchmark.
HTH, Kevin Skoglund