Performance problem about QItemSelectionModel

I have a QTableView which has thousands of rows. When I use QItemSelectionModel to select multiple rows at the same time, my application will become very lag until the selection status of the QTableView is cleared.
However, if I use Ctrl + A to select all rows of the QTableView, the application's performance will not be affected at all.

I make a simple project to test this problem (see attachment). 

The project is a QWidget with a QTableView and a QPushButton. The QTableView has 10,000 rows. Press the button will trigger to select all rows of the QTableView by function selectAll().
The function selectAll() has 4 methods to realize the selection work. Methods 1 & 2 will lead to lag. Methods 3 & 4 works as well as Ctrl + A.

Specifically, if Method 1 or 2 is chosen, the application will be very slow when you tried to move/maximize/minimize the window.

The ultimate goal is to select any rows programmatically, so I must find out the reason for the lag of Method 1 or 2.