Inserting and Removing combobox items

What would be the best way to improve performance on inserting and removing combobox items?
Adding 50 items takes about 14 seconds, and removing 50 items takes about the same amount of time.

The device for testing is an LG VK810 tablet.

The code below is being run after the PDF is rendered in order to be able to correctly find the combobox annotation.

for (int i=0;i<50;i++) {
annot.InsertComboItem(0, "test1", "test1");
}

for (int i=0;i<50;i++) {
annot.RemoveComboItem(0);
}

A better approach shall be to append/remove the item on the last position

for (int i=0;i<50;i++) {
annot.InsertComboItem(i, "test1", "test1");
}