Huge performance loss in Qt 5 when locale-aware sorting a QStringList

In Qt 4 when I inserted some (20 000+) rows into a QTreeWidget or QTableWidget and enabled sorting, it never took more than a second to display that amount of data.

Now, on Qt 5 the very same code executes in 10 seconds on a Core i7.

I attach a sample code (qmake && make) which contains a random string list which gets loaded into a QTreeWidget. Then gets sorted.

In Qt 4.8.1 I get the following output:
Textfile read in 12 ms
Items inserted into the table in 1105 ms
Items sorted in 303 ms

In Qt 5.2:
Textfile read in 6 ms
Items inserted into the table in 58 ms
Items sorted in 9169 ms

This is a huge performance decline.