For a crud style app, the difference in performance between python and C++ is going to be absolutely negligible. The Python version might use more resources, and if you profile it, it might show up with a couple hotspots, but you're still going to have a responsive desktop app if it's implemented properly.

The slowdown is architectural or design based. Sending one HTTP request and not updating your UI until the request has completed fully is going to have _way_ more of an impact on the perceived performance and responsiveness of a native app.