Vojtěch Rylko and Werner Jainek:

Our legacy Things Cloud service was built on Python 2 and Google App Engine. While it was stable, it suffered from a growing list of limitations. In particular, slow response times impacted the user experience, high memory usage drove up infrastructure costs, and Python’s lack of static typing made every change risky. For our push notification system to be fast, we even had to develop a custom C-based service. As these issues accumulated and several deprecations loomed, we realized we needed a change.

They chose to rewrite using Swift on the server.