Maybe I've missed the meme that Ruby isn't fast enough but I've always found the ability to easily write my tight loops in C and compile it as a Ruby module to make Ruby a go to choice _for_ high performance.
The fact that just about every rails performance issue ends in another language is the most telling part of the story. IF your php framework is slow, you might see a BIG speed boost from native PHP, or HHVM, or last resort C. Python has a similar option chain, and that just ISNT there for rails.