Depends on use case, but an huge QTreeView with a custom delegate in faster in python with pure qt (qwidget) than qml.

This is what I said above - you should/have to know how delegates are handled/called in QML otherwise you won't realize where your CPU power is going. It's not a QML issue per se - if you constructed your own stuff with QWidgets and QDeclarativeView that (really) did the same thing, it would be about the same speewise, too.