Please refer the below article and try one of the given methods to fix the slow performance in Lightroom.
Hi! Very sorry for the late response! I've looked over the article and none of the changes seem to have done anything. Lightroom is perfectly snappy during regular use, it's just import and export speeds especially that are very slow.
Is your hard disk close to being full?
No the disk is an SSD which contains both the catalogues and the images. It's at about 400Gb of 1Tb.
Can you try, as an experiment, importing and exporting to a different disk?