If the script was using bash, a noticeable speedup when using dash instead is not surprising. Having used NetBSD's sh for many years as an interactive shell, I find bash on Linux is far too bloated and slow. I modified dash (a derivative of NetBSD sh) for use as an interactive shell as well as a non-interactive one by adding tabcompletion. I like to write quick scripts without having to include #!/bin/sh at the top. I get no benefit from having two shells: one interactive and the other non-interactive.
