this file is an image of 19680 x 17715 pixels. Since it's stored with
indexed colors, the file is relatively small. However to load and
display this image, GIMP will need to allocate about 1.3 GB of memory.
If you don't have enough memory and the tile cache size set high enough,
GIMP can still load this file. But it will need to swap tiles out to
disk and that's what makes loading and working with this image so slow.
There are some things we could do to improve this situation. But nobody
is actively working on such low-level changes at the moment so don't
expect this to improve soon.
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