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
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.