PNM export of very big images is extremally slow
Trying to export PNM (image, which I am not sure selects which PNM format, but I am guessing it tries to find the best one?), is very slow for very big images (approx 10GB uncompressed, 60000x60000). The export process is relatively fast itself, but it takes about 26 minutes for the export dialog to even show up, the one that has only two options: "Raw" and "ASCII". I have no idea why it is so long, as the GIMP already should have all the metadata required and PNM can all be written to disk in a streamed fashion with O(1) allocation that doesn't depend on image size. After clicking final Export the write to disk (actually a network attached filesystem using sshfs), is fast enough about 1 minute, but my server is a bit slugish despite 10Gbps networking used, so that might be the limit of the IO.
During this waiting for the Export dialog to show up GIMP memory usage and CPU usage fluctuates widely.
I have enough RAM to keep entire image in memory. I only have tile cache set to 30 GB tho. I still have 26GB of free memory on my system. (128GB RAM in the machine, but big portion of it is used by filesystem caches and tmpfs).
The image is using 8-bit integers per color (RGB, 24-bit per pixel), without alpha. 23 layers with simple compositing. GIMP is saying 41.8GB for the image resource usage in the GIMP process.
I do not have similar issues exporting to PNG from GIMP.
Using Linux, Debian GNU/Linux, testing, amd64, GIMP 2.10.12-1