If lightroom is exporting raw image files, it is reading from the raw data and applying the develop setting and all the edits applied. Things that are intensive and utilizing CPU, RAM, etc are sharpening, noise reduction, filter, brushing, spotting dust etc, etc, writing rendered image to the HDD. Things that can slow down writing the disk, type of storage, spinning disk, ssd, capacity of disk, % of free disk space.
The timing you are experiencing is expected.
The export process is not just a copy from one location to another as there is no rendered file sitting on a disk waiting to be copied.