.....
But, the best answer is to rewrite your script as a parallel program.
The challenge is: what programming language? Shells can do simple
parallelism via background tasks, if you can break up your script
suitably. I have been beating my head against multi-threaded Perl for
several years, but I do not recommend it. If you want a recent
programming language designed for parallel programming, an obvious
choice is Go. Erlang is older, very robust, and adds distributed
....
You can save yourself some trouble by using a parallel or distributed shell. The two that come to mind are pdksh and IBM's distributed shell dsh.
Then you can write shell scripts in more familiar ways that are concurrent without learning a new language or environment.