Working with images, particularly large ones, is actually a pretty intensive process. For instance, if I open Affinity Designer, t immediately uses about 750-800MB of memory, then opening a single 1.6 MB file makes memory usage jump to 1 GB. If the files are going to /tmp then it's likely because ImageMagick is running out of memory available to it so it's forced to use disk instead, which has larger space to work with, but is much slower.
It may be worth contacting ImageMagick with your configuration to see if any improvements can be made.