I’m not sure if you can run several threads but you can certainly run several processes.
You can use Slicer’s CLI module interface to synchronize execution and pass data between the main process and the parallel processes. CLI modules can be implemented in Python or C++.
We plan to have native Python3 support and ability to install any Python packages within a few weeks. This will allow you to use any of the native Python multi-processing methods. It won’t make things faster or simpler, the only difference is that you can use pure Python mechanisms instead of Slicer-specific ones.