Is it possible to make it faster?
In Photoshop doesn't matter if you have a graphics card or not, big or small dimensions, it's pretty fast to fill areas.
But in Krita I've experienced long lags when working on a big dimensions work.
You can test that in a blank document, creating various areas with any brush tool, and then with the filling tool start clicking around. It takes seconds (not milliseconds) for the areas to be filled in the sequence you clicked.