When you preview the whole screen you get in fact a window telling you “render completed in …” and the you can save the result in a few seconds.
Unfortunately the given time is very rough after it exceeds one hour, e.g. just saying 6.5 hours.
Anyway, I now (quite randomly) selected two images, a low res one from WhatsApp with terrible quality and a size of 785x1000 pixels and a “normal high res” 12 MPix image from the iPhone with the standard resolution of 4032x3024.
I primarily did want to incorporate higher resolution images but seeing the time needed here for the iPhone image on my machine quickly lost interest in that 
For low-res and low-quality WhatsApp images Redefine can do wonders:
Redefine gave me this really good result:
Recover was much less impressive but had much longer processing time.
I created a thread with the results and the used images for benchmarking:
Feel free to add your numbers…
