I've noticed a problem when importing Nikon Z9 RAW files into Lightroom. Importing directly from a CFExpress Type B card is much slower than expected.
To illustrate that this is not a problem with file size, I also shoot with a D850 (identical resolution) and have used it for years with no issue. Furthermore, I use a compressed RAW with the Z9, so the files are significantly smaller than what I'm used to (approximately 33MB/each vs 66MB/each).
To further illustrate that it seems to be a problem with Lightroom Classic's import, I've found a (pain-in-the-butt) workaround that completely negates the slow speed. If I drag the files straight from my card into a folder created for an event, then use Import->Add, Lightroom is extremely fast. The issue only arises when importing directly via Lightroom.
When using my workaround, a moderately full card (say 32GB of files) can be transferred and added to my LR catalog in approximately 4-5 minutes. This includes moving the files off the card into the new folder. When I attempt to do the same using the normal method of importing directly into LR, the process takes between one and two hours.
I'm using a card reader. It's fast when transferring files manually. About 4 minutes for 32GB of photos.
I'm attempting the method you suggested (copy files to a temp folder on my internal SSD and import/copy from there). It's about twice as slow as moving files manually and import-add, but an order of magnitude faster than importing directly from the card. I'm baffled as to what that might suggest.