Haaaaaa… I missed that… You didn't pass the pointer to the QImage data...

@mjsurette That did the trick! I also removed the dataChanged() calls and now the operation executes instantly even with thousands of rows. It also seems there aren't any side effects, apart that I have to call again fetchMore() immediately after select(). I can now move on and implement some other batch operations like this without worrying about speed. Thank you!

@Adrianos How about using setSelected ? Since you already have the QTableWidgetItem .

@MaestroMaus Nevermind; found it. Qt has a custom way of downcasting QVariant subclasses. I can just ask the type via a standard method.