Joel wrote about Ruby's performance, and DHH replied with a post showing how he outsourcedhe performance-intensive functions. To note, my only experiance in Ruby is writing very few Watir tests. So I can't really say anything about Ruby's perfomance first hand. I agree with DHH that this is a good thing, but I wonder about how to handle this in situations where the performance critical part is something that is core to the business logic.
I'm not talking about general stuff like image resizing, encryption or bayesian filtering (which I think you are crazy if you are writing your own for production). What I am talking about is that an application that is not mostly data entry and pre-calculated reports (a good example of which is a bug tracking system).
Let us assume a package delivery application, which let the user choose the route that they can send their packages, of the top of my head, you need to calculate cost (time & money) of moving the package each route while taking into account service level agreements, legal responsabilities, contract issues, past history, validity dates, etc. This get complex very fast, and the amount of data that you need to consider is fairly big, especially if you need to consider business rules like (if customer send more than 10 package a month for the last 3 months, give 4% off, etc).
You can do this on the backend, to pre-calculate the most common ( or all ) routes and their costs, but it may very well be that you simply have too much parameters to do this pre-calculation (or are prevent for business reasons).
Assuming that I had a web application in Ruby on Rails, and I wanted to make the choose a route page work, how would I go about building it? This is mainly a CPU bound task, with a limited amount of data to fetch and process, but I can't easily drop down to C for this task. This is a task that involve quite a bit of business logic (just finding out if a contract is valid or not may be a complex process, for instance), which I would have to duplicate in C (I may be able to hand the data to the C program from Ruby in a usable form, but I doubt it) in order to gain the neccecary performance.
So, given this scenario (and, of course, assuming that doing this in Ruby is not performant enough), what are the options that I have?