Beats the hell out of Python or Ruby in that department. But regardless, absolute speed isn't the bottleneck in most situations, and in those cases trying to force JS will end you in a world of hurt.
Like most things in life, use it in moderation, and you'll end up much happier. But if you take absolute stances on things, you are only hurting yourself.
Then they rewrote everything from scratch because as it turns out, javascript on the server side is still pretty damn awful performance wise.