It seems QComboBox with an item model is query the data() method even while hovering the mouse over the box or the drop down. So simple moving the mouse over the combo box produces 100 and more calls on the item model. Normally, this is not a big issue. But:

QComboBox -> ExtenderAdaper -> QSortFilterProxyModel -> DataToItemModel -> TreeToFlatAdapter -> OriginalDataSource.

Would be great to optimize QComboBox so that new data is only requested from the model if the data changes. For some reason, the data model provides data change notifications. One can write another item model adapter which is caching all data, but it would be nice to solve the "problem" at the source.