Enable instant preview
Learn how to enable and work with Instant Preview. Watch the demo & read the steps:
Open Partial processing Dialog.
If the image is large, select an area, using the handles.
Check the Instant Preview box. Then click the Instant Preview button.
The image will open in Split View. The Partial processing icon will be active. Once you make changes to the area, the tiny progress bar will appear and indicate the progress of processing.