I am on a fairly beefy 2015 Retina iMac (highest specs). I am importing photos from my thunderbolt connected NAS (QNAP 1282-T). When doing so, Lightroom comes to a grinding halt. It takes about 15-20 minutes to import 40 photos, during which the interface becomes unresponsive. It sits at "importing 0 of 40" for 5 minutes and all of a sudden it jumps to "9 of 40" about 5 minutes later.
Things get even worse if I start another import while the first one is running (I am importing in batches to add to new albums during import). When I do this, and it finally gets to the import preview screen, it only displays about half of the photo thumbnails and claims "Preview missing" on the other half, all while the interface is completely frozen. When it unfreezes, these "Preview missing" spots start filling up with actual thumbnails until it freezes again.
My NAS performs extremely well, it's a high-end NAS with SSDs, connected directly to the iMac with Thunderbolt. When I copy files from the NAS at the same time Lightroom is having these issues, they transfer in mere milliseconds. So I am sure I can rule out the NAS as the culprit.