Indeed! Very unlikely...

I greatly appreciate the various answers in response to my question. I am not a computer scientist and parts of those answers are well above my level of understanding. I have been a user of Mathematica for some time, and was in the process of preparing an introductory talk for students when I thought I might benefit from other users experience, looking for a few examples where the power of functional programming could be best shown, both in terms of operational power (i.e. speed of execution, import and export facilities, storing results - not only programs or notebooks, but also intermediate results) and of course the quality of the language and how easy it is to get programs up and running, testing code, combining text, code, images, etc. Far from me the idea of starting a flame war, as Matthew says. I am essentially a Mathematica-oriented practitioner, and rather than trying to promote or sell the advantages of the language (in which I firmly believe, perhaps because I practically know nothing else), I wish to provide clear-cut examples where functional programming is faster to write, easier to understand and very good in performance terms (and also, as Frank says, easier to debug) than procedural or imperative languages. All this said, I must add that reading the replies has been extremely useful and I sincerely thank you all. Szabolcs provides a lot of ideas and useful examples, although some of the references he gives were hard to understand. George's position, I fully agree with. And Henrik, thanks for the link to Rosetta Code: quite interesting.

In general, using built-in Wolfram Language functions is going to be faster than rolling your own. This is particularly true for any type of for-next loop construction. The trick is to understand what you want to do in terms of these functions. The utility of Wolfram Language is that you can mix programming styles to match what you want to do.

Also, the "best" code is code where you have to write the least. I have seen this mantra for other languages (objective-c and Swift -- I am/was a Mac developer). The people writing the code behind Wolfram Language probably spend a lot more time understanding the deep details of the computer science behind algorithms, even if the code is eventually written in c or some other imperative language. In a sense, these people are your collaborators, and it makes sense to use the resource.

Just as there are beautiful and ugly proofs in Mathematics, there is beautiful and ugly code. I have written plenty of ugly code, but my experience is that beautiful code is generally more robust and easier to maintain (and understand), and it is worth a minor performance hit. It was worth the time and effort to rewrite the code to make it beautiful.

It also depends on what you are using the code for. For a single use application, the faster you can write good code, the better. For "production" code, robustness wins out over speed in almost all cases. It is only when you are pushing the envelope that execution speed is primary.

How easy one coding style or the other is to debug depends on the background of the coder doing the debugging! If a programmer has spent years writing and debugging procedural coding but only a relatively short time writing functional code, then it may indeed be quicker for that programmer to debug code that is procedural, even though it is much longer (with lots of nested loops). I have seen that phenomenon in action, including by students coming to Mathematica after extensive experience with a procedural language. (I saw the same thing regarding array-oriented programming in APL and J vs. procedural programming in "conventional" languages.)