When you say "Wipe the iMac completely" are you talking Everything, OS, Programs, files and then a reinstall of everything. Or just LR.
Just because other people using a similar systems aren't have the same problem you are having doesn't mean there isn't a problem with the hardware in your particular system.
The import process, especially from a hard drive using the Add option in the import dialog window and not creating any preview should be extremely quick. All that is happening is LR is making a Reference, a record, of them in the LR Catalog file, A Database file, of where those images are stored on your drive.
When I got back from my trip in January I have about 1900 images that I had copied over from my notebook to my desktops hard drive and the import took maybe 20 seconds. And that was on a 7 year old Windows 7 computer.
Thanks for the advice, sync is off already so that wasn't causing the issue.
I haven't ruled out a hardware issue but nothing else on the machine is exhibiting any sort of problem, which is the most frustrating thing. And exporting is really quick. I don't use ADD though, I have always used COPY and moved the files to a new location from the memory card at the import stage. That typically doesn't take long at all usually, even with 45mp raw files. But that's always been my workflow even on my last system which was 5 years old and involved an external spinner hard drive to copy too.
One thing I haven't tried is to connect up my old spinner and see if that works better, so I can rule out an issue with the Extreme 900 SSD I'm using. But again, copying to and from that isn't really exhibiting any slowness in any other scenario outside of Lightroom either.
And yep, the plan was to wipe the machine and start again. I've reinstalled Lightroom twice now with no success.
UPDATE: I think the problem is solved. Flying out this afternoon for a week away but it was bugging me so much I tried one last thing, I reformatted the Sandisk Extreme 900 that all my images are stored on, put them all back on again and then tried to import 93 raw files from the D850. This time it took about 10-15 seconds!
So it would appear the issue was with the external drive. Have to head to the airport now so I won't get to fully test until I'm home with images from the shoot but it seems like it should be fine now.
Thanks again for the advice.
Again another hardware issue. Glad you got it working. Have a good trip.