The script doesn't write the images when the images output path is blank, as you show. However, it still has to rasterize the styles and trim each layer in order to know the size of the images that aren't being written. Photoshop is unfortunately slow at doing this, so it still takes a while even though no PNG files are written.
We've improved this in the latest script (6.9) so it processes much more quickly when writing only JSON (ie when the images output path is blank).
