Rotate by EXIF orientation is slow and changes unrelated pic

[Matze]

Hello everybody,

I wanted to automatically "mass rotate" a lot of my pictures regarding the EXIF orientation tag,
rebuild their thumbnails and reset the orientation flag.

There are a lot of pictures in a directory and I selected all of them - even those, which have a correct orientation and don't need to be rotated.

XnView rotates them - but is very slow and it seems that it also changes the files which don't need a rotation
(either because they don't have the orientation flag set or because the orientation is already correct)

I understand that it needs to read the EXIF data of all files - but why does it change _all_ files and not only those with "wrong" orientation?
And why is it so slow doing this (I waited more than three hours for about 1000 files from which only about 10% need to be rotated).

regards,
Matthias
P.S.: I didn't find that as batch action in XnViewMP nor in XnConvert - is this correct?

[xnview]

Which function do you have used? Exif rotation?