I think these days for web development its not really a question of productivity vs perf, since you can pretty much attach your app from whatever language into

I think these days for web development its not really a question of productivity vs perf, since you can pretty much attach your app from whatever language into your load balancers / db clusters / caches etc, and you get solid, scalable performance (that's been my experience with python, ruby, and php at least). There's really a great degree of freedom to just choose whatever lang you and your team are most productive in and just go from there.