It is possible for plugins to create threads using Python's threading module or Qt for Python threading related classes.
This can be useful to perform background processing or IO operations while Designer is running.
It is important to note that most classes and methods in Designer's Python API can only be called from the main application thread. As such, if you want to make any modification to any graph that is currently opened in Designer, you must make them from the main application thread.
One possible solution is to use QThread and Queued connections, like in the following example: