Flask/Python blows Node out of the water where performance is concerned. That shouldn't be a surprise to anyone.
Flask has a significantly faster ramp up time than Tornado as it is designed to be simple.
As far as load balancing/performance, you front end it with something like NginX.
You can process things out of band easily enough with Flask, but I wouldn't evangelize it so much to say that it's the best solution for every problem.
Personally, I like to keep things as simple and as logically separated as possible until it's absolutely necessary to change. Flask is one of those frameworks that doesn't present a lot of mystery.
My biggest problem with node isn't performance - for that it's fine for most use cases that I deal with. It's that you can get perl-level convoluted and sloppy very quickly and easily.