For me this takes around 15 seconds.
In order to create such large images we have to split up the viewport into several rectangles. Each rectangle is rendered separately, and then stitched together.
When the skylight shadows are enabled we need to increase the size of each rectangle significantly, otherwise it would look really bad when stitching everything together into a final image.
Having said that, I can take a look sometime this week whether this can be improved in any way.