[racket] Performance help

I have been trying to translate Norvig's famous spelling correction program
into Racket. Norvig's Python version is:

http://norvig.com/spell-correct.html

and my Racket version is

https://github.com/jmoy/norvig-spell/blob/master/racket/norvig.rkt

On my system the Racket version takes about 31s while the Python version
takes 12s to on a test set. These measurements are done with command-line
Racket. The README of my GitHub repository describes the interface of the
programs and the test files.

I would appreciate any advice of how I might improve the performance of the
program without fundamentally changing its structure. As a beginner I would
appreciate any other comments on style as well.

Regards,
Jyotirmoy Bhattacharya
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