At my previous job a colleague at one point had the use case of highly detailed scans of cell tissue where he'd like to be able to both zoom out to see the structure the entire tissue, but also be able to zoom in down to individual cells. The pngs were gigabytes in size and he was a bit worried about the idea of using lossy jpgs since they might have messed up the tiny cell blotches.

We first tried to find some sort of map-library based solution that converted the images into a set of tiles, but eventually I found Viewnior which somehow seemed to be able to handle the pngs relatively well compared to any of the other image viewers we tried at the time (which all would choke on the giant sized images)

https://github.com/hellosiyan/Viewnior

I know that that's a very niche use case, and I have no idea how Qimgv compares ("fast" is not the same as "scales well with really large images"), but I thought I'd share in case anyone else has had to deal with this problem.