SIMD roughly means CPU instructions that do multiple things at once. Making use of them can make algorithms faster. The rust compiler actually uses them automatically, but it doesn’t for newer SIMD instructions like SSE4.2 & AVX2. SIMD Feature Coverage SSE2 100% SSE4.2 99.1% AVX2 89.2% AVX512F 8.73% Steam survey October 2022 Rustc can’t use them by default because not every CPU will work with them. But these days most CPUs do support these newer instructions and so certain tasks can be a lot faster for a lot of people.