imagerotate has special intern functions when the rotation angle is a multiple of 90 degrees, it actually only flips the image. The operation is quite fast as it is a two nest loops with a little copy operation. However it looks like the manual has a little mistake in the function description, which can lead to confusion.

The problem comes from a part mentioning the scaling down of an image to fit into the destination image. With a flip of 90 degrees, though, the resulting image contents will stay the same size as the original - the image only flips, not really rotates.