There may come a time when you need to improve the performance of your software beyond what can be accomplished simply through good design, by using the right data structures and applying the correct algorithms. Modern operating systems, CPUs, and compilers are remarkably good at handling most of this job for you, but every once in a while, you’ll have to go deeper.
This topic deserves an entire book of its own, but I’ve distilled the key points down to their essence to provide a good jumping-off point. Rust’s safety, concurrency, parallelism, asynchronicity, and SIMD features make it an exceptionally compelling programming language, all packaged as pure, open source software.