concurrency != multi-threading.

Crystal's concurrency is based on fibers (green threads) which works very well even on a single thread (it scales to multiple threads, as well)

And for many use cases with high parallelization and little shared state (including web applications such as a search engine), multi-threading isn't necessary. Synchronization overhead is a serious performance killer. You can get much more performance out of running N processes with a single thread instead of a single process running N threads.