Gimp and Darktable are very different beasts. Darktable is tailored to work on RAW pictures coming directly, unprocessed from your camera. Gimp cannot do that and only works on compresseda and/or rasterized images (thinkg JPG or PNG).
Another big difference is that Darktable does non-destructive changes: you have a history of operations that you applied and you can rollback to any step. None of the steps directly modifies the pixels of the image, instead the program computes the changes made by all of the effects in the history for every pixel in the image, then shows you a preview (or exports it).
Handling a big RAW file and all the modifications that you've done is an intensive process, and that's why it requires a beefier computer than Gimp for instance. There is also OpenCL support, to leverage the power of your GPU.
Another big difference is that Darktable does non-destructive changes: you have a history of operations that you applied and you can rollback to any step. None of the steps directly modifies the pixels of the image, instead the program computes the changes made by all of the effects in the history for every pixel in the image, then shows you a preview (or exports it).
Handling a big RAW file and all the modifications that you've done is an intensive process, and that's why it requires a beefier computer than Gimp for instance. There is also OpenCL support, to leverage the power of your GPU.