Your original post was correct Gini - Tiari is talking about something else, rendering too few pixels and then resampling to make the image larger in its pixel dimensions. As long as you have Resample unchecked in the Image>Image size dialogue in Photoshop then that won't happen, all you're doing is changing the size of the pixels at print time. And the 72 PPI image will take exactly the same time to render as a 300 PPI image with the same pixel dimensions, PPI has no bearing on render time, quality, or file-size as long as the pixel dimension are kept constant - which you are doing.