Cygwin bash isn't slow either. The problem is a typical bash script isn't a series of bash operations, it's a series of command line program executions.

For example, someone might do something like this (completely ignoring the need to quote in the interests of illustrating the actual issue, forking):

    for x in *; do
      new_name=$(echo $x | sed 's/old/new/')
      mv $x $new_name
    done

Instead of something like this:

    for x in *; do
      echo $x
    done | sed -r 's|(.*)old(.*)|mv \1old\2 \1new\2|' | grep '^mv ' | bash

This avoids a sed invocation per loop and eliminates self-renames, but it's harder to work with.

Of course the code as written is completely unusuable in the presence of spaces or other weird characters in filenames, do not use this.