If concurrent processing is a way of structuring our programs that has performance implications, parallel processing is a way of getting better performance that has implications in how we structure our programs. Although often conflated, concurrent processing and parallel processing are really different solutions to different problems. Concurrency is good for expressing programs that involve different tasks that can be, or must be, carried on at the same time. Parallelization is good for doing the same task many, many times, all at once.
It used to be that the easiest, and often best, strategy for improving performance was to go on vacation. Moore's law implies that processor speed would double approximately every 18 months, so in the 1990s, we could go on vacation, return, buy a new computer, and our programs were faster. It was magic.
Today, we're no longer under Moore's law, however. Instead, as the saying goes, "the free lunch is over." Now, processor speeds have plateaued...