Thought I would share my results, my project uses a lot of pages of comboBoxes (100's) with lists up to 2000 items long.
I have suffered from very long executable startup times (40+ seconds) and have narrowed the issues down to the comboBoxes (currently Qt5.12).
When building the combo list using comboBox.addItem() seems to slow things A LOT !!

My successful result is to build the list using QList<QString> and then use comboBox.addItems(list);
This has speed up start time and page load times by a factor of about 5x