Do you have any external drive connected to your computer? If yes, then please disconnect it and then try to import.
Check the below article as well to optimize its performance:
Please let us know if it helps.
What size in megapixels (not megabytes) are these photos?
When the entire computer locks up, this is often due to a hardware malfunction, such as a fan malfunction causing your computer CPU to overheat.
>Imports are RAW , copy as DNG...
Try to import as plain raw and don't convert to dng. I vaguely remember about a bug with this on many core systems in Lightroom 5.7. Don't know if that's the problem but worth a try. You can always convert to dng later.
>I did increase the camera RAW cache to 75GB per another post I read-
That shouldn't make any difference for this part of the process. It's just going to take up hard drive space.