Limited myself to two hours. Rough, but good enough for my needs..
I never cease to be surprised by how badly my image editor (GraphicConverter) runs with large images (in this case 10,796 x 4377). Almost unusable on a final generation Intel 16" MacBook pro specced out with 64GB RAM and Radeon 5600M with 8GB RAM. Should have waited for the Apple Silicon. Sigh.