I believe I have an easy way of making NPrinting 17.4 (June 2017) suddenly inoperable or rather very, very slow for a long time.
1. Edit a report with NPrinting Designer, e.g. an Excel template
2. Accidentally or deliberately compose a template that will crunch through a large or complex amount of data, e.g. via Page/Level iteration
3. Perform a report preview
6. Realize you have previewed a long-running report, with no way to stop it, and have a strong desire to abort it
7. Manually kill the NPrinting process and the companion Excel process.
8. Although the web console remains responsive, you are now unable to run any other jobs. This was my experience, repeated on a number of occasions.
As far as I can tell, the original NPrinting preview process continues running on the server consuming lots of resources, despite the NPrinting Designer getting terminated.
If I am right about aborted preview jobs continuing to run to completion on the server, there is probably an easy fix for this: surface these NPrinting Designer preview jobs in the Task Executions admin web console, the same way Publish tasks get tracked there and also connection cache-generation tasks. These tracked jobs can be manually aborted if desired, and that's definitely what needs to happen with accidental long-running preview jobs. All jobs need to be manageable and able to be aborted if they are long-running -- especially long-running Designer preview jobs.