I am using LR CC 2015.14, a fast card and a good reader. This common problem of LR being slow to import to import has finally settled in here. The computer is a Mid 2012 Mac Pro with 64GB of ram running High sierra 10.13.6. Everything is up to date according to Apple and the Adobe Cloud. A Promaster UDMA 7 128GB and a Sandisc 128GB SDXC card were both tried. I tested the card/reader combo by uploading to a folder on the desktop and that was fast enough. Going straight to LR CC is still slow. I have read many posts on this topic and tried unchecking options (auto write to XMP was not checked nor embed previews) and deleting 3 files from preferences to no avail.
This is a show stopper and I really do not know where to go from here. Any fresh ideas?
Reading more advice I upgraded to LR Classic 9.3 with no change. The Canon 5D Mark IV files are uploading at a rate of about 1 every 2 minutes. Uploading straight to a HD takes 2.5' for 8 files. Uploading to LRC on my MacBook Pro took 20 seconds. That's certainly more of what I expect and have been seeing for years on any LR version. There has to be an explanation since there usually is.
Some mod can delete this posting at will. As it turns out, 8TB remaining on a 4TB drive is just not enough. I tried uploading to a more lightly loaded 1TB drive and it behaved as expected. So problem solved.