I've rarely found that GUI code is ever the bottleneck. Usually it's only a problem for mousemove handlers and code that runs directly in the event loop. Most GUI apps don't even have those, unless you're writing a game.
The big problem with Python GUIs is usually that the GUI framework is often written in Python. (Or it's that things like network & parsing code gets written in Python.) The framework does a lot more heavy lifting than the app does - in particular, it's responsible for converting the system events into clicks, drag&drops, submissions, etc. for the app, and for rendering components into bitmaps. If the framework is written in C++ and just exposes a Python API, there's no problem. That's how wxPython and PyQT do it.
The big problem with Python GUIs is usually that the GUI framework is often written in Python. (Or it's that things like network & parsing code gets written in Python.) The framework does a lot more heavy lifting than the app does - in particular, it's responsible for converting the system events into clicks, drag&drops, submissions, etc. for the app, and for rendering components into bitmaps. If the framework is written in C++ and just exposes a Python API, there's no problem. That's how wxPython and PyQT do it.