I am using AP 1.6.7 on a 5K iMac, current OS. I am working on images of 20000x15000 pixels at 72dpi and around a dozen smaller image layers averaging about 1000 px. The small images(SI) come from jpegs. Open the SI by dragging it onto AP icon. Opens fine, shows as a pixel image, develop time instantaneous. Drag the same image onto my large background. Shows as an image layer, rasterize to pixel. Select the layer and develop. Develop time goes to many seconds. As the number of layers build up, develop time for any individual layer increases, until it reaches many minutes to never. Select a SI layer, copy and create new image from clipboard. Develop time, with no background, is long. Export the SI to jpeg, reopen in AP and develop time is instant. It would appear that once an image is rasterized, develop times take much longer. There is more detail, but the increasing develop time is the main issue here. Otherwise, AP is working just great for me. I shoot everything HDR and AP tone mapping is a killer feature.