[racket] Why no string function to replace substring based on content in Racket

I wasn't able to find in the Racket documentation a string function
that replaces substrings in a string based on the content of the
substring.  Something similar to Python's string replace method .
s = "abcdefabcdef"
s.replace("abc" "123")  -> "123def123def"

But more surprising, I also wasn't able to find such a function in
SRFI13 which is supposed to be a comprehensive set of operations on
strings.
Its string-replace function is based on position not on content of the
substring.

Is there some functional programming reason for not having such a
built-in function in Racket/Scheme, or is the idea that you use
regular expressions to do this.
My understanding is that regular expressions can be quite expensive.
I have Python programs which convert Indian language book length
etexts from one transliteration scheme to another so I'm calling the
Python  replace function hundreds of thousands of times per e-text and
doing multiple replaces with each call.

I'd like to convert these programs to Racket, but using regular
expressions might be too slow.  Is  writing a C foreign function.
extension my only alternative or is there such a function in some
Racket package..

Thanks,
Harry Spier