Thank you for your question.
The impoundment size index (ISI) tool is an extremely computationally intensive tool, especially during the flow accumulation process. As a result, it is likely that the parts of the tool and your python script will run slow.
The ISI tool is designed to be as efficient as possible. In the tool itself, every part of the tool that can run in parallel already does. However, there are some parts in the code that cannot run in parallel and as a result run time increases. In addition to this, the tool is extremely memory intensive.
Unfortunately, this is just the nature of the algorithm. Unless you are using a very powerful super computer, it is very likely that you will want to run this tool sequentially on your list of input DEMs.
The only other thing I can think of is the Multiprocessing python package. This package will allow you to send tasks to the various cores on your machine. I have had some luck with this package before, but I don’t know how useable it will be in this scenario. It is likely if you try run this tool on multiple DEMs at a time using multiple cores, you will either overload your cores or run out of memory quickly.