As already mentioned in neighboring comment, these languages give you control over data layout. But what's really powerful - is that control over layout opens u

As already mentioned in neighboring comment, these languages give you control over data layout. But what's really powerful - is that control over layout opens up a possibility of using SIMD instructions, which by itself may have 10x speed up over optimized scalar code in the same languages, speed up over JS is, of course, greater.