title: QCombobox works very slow with QSqlQueryModel with large model
body: <p>I have few comboboxes with very dig data sets within ~ 100K rows and more. I tried it with <code>QStandardItemModel</code> - works fast enough if model is preloaded, also model loading takes few seconds if performed in separate thread. Tried comboboxes with <code>QSqlQueryModel</code> without threading to improve performance but experienced it works much slower than <code>QStandardItemModel</code> (in our project <code>QSqlQueryModel</code> works very fast with such amount of data with <code>QTreeView</code> for example). What could be the problem here? Is there a way to speed-up combobox, some parameters?</p>
  
  <p>P.S. Suggested by Qt doc <code>QComboBox::AdjustToMinimumContentsLengthWithIcon</code> does not speed things much: dialog with such combos starts too long and exits 10-20 sec. <code>AdjustToMinimumContentsLength</code> works a little bit faster but anyway delays are too long.</p>