Software that takes too long to perform its functions rapidly becomes difficult and unpleasant to use, thus making it both less useful and less usable. For real time systems, of course, performance is even more of an issue and is an important aspect of correctness.
The other aspect of performance is the amount of memory and storage that is used. These are less of a problem nowadays than they used to be, but can still be a limiting factor. Even today’s large disks can eventually run out of space, and some systems run very slowly if at all if not enough memory is present. The system then becomes effectively unavailable for use.