Here's the way I look at it: computing is becoming exponentially cheaper, but good developers are incredibly expensive. Anything that makes the later more productive is overwhelmingly likely to be worth an increase in hardware cost (to cover up lost performance).
The problem is that performance is an academically interesting problem to us geeks. We love to optimize, make quicker and oh-so clever.
I think this sharp antagonism between productivity and performance is a fallacy. It only works in the extremes. Making something slow doesn't automatically make it more productive to use. For framework writers to give some thought to performance issues does not make users of that framework less productive.
Also, if you look at what just a few selective type hints in clojure can do to performance or how terse Scala code is, you have to come to the conclusion that performance and productivity can go hand in hand.
I guess you could take that viewpoint if you're planning on releasing your product 3 years from now (for example, that might be okay for a game developer with long product cycles), but 500ms to render a page is slow today. So, is the idea to wait 5 years so your page renders in a reasonable amount of time?
The problem is that performance is an academically interesting problem to us geeks. We love to optimize, make quicker and oh-so clever.