1) Note that the post doesn't sell its speed; all it says is that they are interested in a high-performance Ruby 2) Python is a very similar language to Ruby,

1) Note that the post doesn't sell its speed; all it says is that they are interested in a high-performance Ruby 2) Python is a very similar language to Ruby, and Pypy already runs python very rapidly. This doesn't guarantee that the Ruby interpreter will be anywhere near as fast, of course, but it does give evidence that it's possible. 3) As kingkilr notes below, they've taken into account your argument and they believe they've implemented enough of the language to be confident that they can run it rapidly. No reason you need to believe him, but it's worth listening to.