I have done both in Ruby, and addressing it was not a big problem. E.g. my MSc. involved doing a lot of statistics and image processing in Ruby, and solving the performance bottlenecks meant rewriting about ~30 lines or so in C using Ruby Inline. Later I did map tile rendering on demand handling thousands of layers uploaded by customers in Ruby. Both using 1.8.x, btw. - far slower than current versions.
It took more CPU than if we'd rewritten the core of it in something else, but it let us iterate the rendering engine itself much faster, and most of the expensive computations were done by extensions in C anyway (e.g. GDAL and the like).
Of course you can find areas where it's still not viable, but my experience is that if you start by prototyping in whichever high level language - no matter how slow - that is most productive for you, you'll inevitably find you need to rewrite far less than you might think to get it fast enough. But more importantly: The parts you end up rewriting will very often be entirely different parts than what you expected, because being able to iterate your architecture quickly tends to make you end up in a very different place.
It took more CPU than if we'd rewritten the core of it in something else, but it let us iterate the rendering engine itself much faster, and most of the expensive computations were done by extensions in C anyway (e.g. GDAL and the like).
Of course you can find areas where it's still not viable, but my experience is that if you start by prototyping in whichever high level language - no matter how slow - that is most productive for you, you'll inevitably find you need to rewrite far less than you might think to get it fast enough. But more importantly: The parts you end up rewriting will very often be entirely different parts than what you expected, because being able to iterate your architecture quickly tends to make you end up in a very different place.