Hi there Thomas
Just to let you know, you are definitely not on your own with this problem. We have an important image to restore, including white marks when it got dragged over a dirty floor.
We spent around an hour “removing” 469 of those marks and, all of a sudden, we got exactly the same thing as you and the image refused to show in the Ian editing window.
This is a RAW file that has had adjustments made to make it B&W along with a couple of geometry adjustments.
I am using PL7.3, because PL7.4 has another bug that I can no longer remember, so I reverted. But the problem of Retouch getting slower has been around for several versions. The only kludge we found was, if you can remember, to export to TIFF as soon as things start to get slow and then restart work on the exported TIFF until the same happens again, where we went through the same “wash, rinse, repeat” cycle.
I tried to report this but when support asked for the image file, it is a 45Mpx Nikon NEF file, which was too large to upload with the report. They have given me a separate upload URL, so I shall try to upload there.
We too are also seriously considering changing to Affinity Photo for retouching but it means relearning the reverse way of selecting the source before the destination, which isn’t as efficient as the PL way.
We are using Ventura 13.6, although, as I’ve said, the problem has existed for years.