[racket] Request feedback on a boggle solver

Hi,

I've been reading HTDP and I'm about half way through. To apply what I've
learned in the book to a bigger exercise, I decided to write a boggle
solver.

You can find the code here:
https://github.com/jduan/boggle_scheme/blob/master/boggle.rkt

Any feedback is greatly appreciated. I'm specifically looking for feedback
on:

1. Did I break the problem into right components?
2. How can I make the code more idiomatic?
3. Why is code so slow? It can take minutes to solve a 4 by 4 board. The
same board can be solved by a ruby program I wrote in much less time, like
a few seconds. I know recursive functions are a big reason but is there
anything?

Thanks,
Jingjing
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