Before running / importing any other Twisted code, invoke:

app = QApplication(sys.argv) # your code to init QtCore
from twisted.application import reactors
reactors.installReactor('pyside2')

trial --reactor=pyside2 twisted

If you're writing a conventional Qt application and just want twisted as an addon, you can get that by calling reactor.runReturn() instead of run(). This call needs to occur after your installation of of the reactor and after QApplication.exec_() (or QCoreApplication.exec_() whichever you are using.

reactor.run() will also work as expected in a typical twisted application

Note that if a QApplication or QCoreApplication instance isn't constructed prior to calling reactor run, an internally owned QCoreApplication is created and destroyed. This won't work if you call runReturn instead of run unless you take responsibility for destroying QCoreApplication yourself...

However, most users want this reactor to do gui stuff so this shouldn't be an issue.

Performance impact of Qt has been reduced by minimizing use of signaling which is expensive.