use multiple threads / CPU cores to generate thumbnails
If a directory contains a large number of high resolution images, for example, from a DSLR then it can take a very long time to generate the thumbnail images.
I'm using a workstation with 64 cores and I notice gthumb is only using 1 core for generating thumbnail images.
My system is running Debian 11, gthumb 3.11.2-0.1