[quote author="nullstellensatz" date="1421370989"]The operation that is taking too long is a library function call that I have little control over.[/quote]That's unfortunate. Any chance of getting the library's author to provide a better API?
[quote]I am looking to understand why I cannot terminate() QThreads in PySide.[/quote]Not sure (I don't have Python experience), but judging from your error message, I'm guessing it's related to how PySide handles the "Global Interpreter Lock":https://docs.python.org/2/c-api/init.html#thread-state-and-the-global-interpreter-lock