Settings (or actually WU) has to take into account if Windows has been patched.
FOD packages are created only once by MS, when a new Windows release is in the earliest build stage. Afterwards, the FOD file will never change, even if Windows has. There can be later patch updates to the FOD.
In the best case, a FOD is integrated into your current image. Then DISM just needs to hard link files, update the registry, and possibly reboot.
For optional FOD's which are found mostly online (outside of the Languages & Optional Features ISO), WU has to download the feature, integrate it into your host image, and patch it. That's all before enabling that feature.
So yes, all those steps take longer than what the reported size of the Feature update leads you to think. WU has to do it right, otherwise you would be forced to re-install the last CU all over again.