Typed Racket is more ambitious than other attempts at adding types to an underlying untyped language. Namely, Typed Racket guarantees that typed code is never to blame for certain contract violations, and, if any such contract violation happens, it will be properly traced back to an offending piece of untyped code. This is what makes gradual types gradual (as opposed to merely optional), alas, it is also what has been found to have unacceptable overhead.

Relevant paper and talk:

http://www.ccis.northeastern.edu/home/types/publications/gra...

https://www.youtube.com/watch?v=1u1JGwmW0IQ