Unityped vs multityped

Performance Warning: Matrix values are arrays, as exported by math/array. The same performance warning applies: operations are currently 25-50 times slower in untyped Racket than in Typed Racket, due to the overhead of checking higher-order contracts. We are working on it.
http://docs.racket-lang.org/math/matrices.html

Still not convinced that unityped languages suck ass?

Why should I care when unityped languages are slower? The vast majority of my code isn't running high performance algorithms. If I ever need high performance code, I'll probably write a  library of functions in C (if they don't already exist) then use that library through a foreign function interface.