Face it, often the available processing time is very much finite, and the slower version just doesn't cut it.

And the same holds for memory. Your system has only so much of it, and for applications with many small objects most high level languages are very memory-inefficient if written in an idiomatic style. This can easily amount to a size factor of 20x in what's processable at all (disregarding execution time). Add garbage collection (which can "drown" processing) and the factor might even be higher.

Try adding a few Integers to a Java map and you will be surprised how inefficient it is.

I suppose in many applications it is. My career has pretty much been in Web apps and generally most work is I/O-dominated

"If you want to write efficient software, which is certainly not always necessary, ..."