4K video is pretty demanding on a CPU particularly if it is high bitrate. So possibly one hour import time for a 15 minute clip is not abnormal. If your computer has the slower i5 processor versus the faster i7, that might cause slower import speeds. As you mention, you certainly could try importing from your high speed LaCie drive and see if it makes a noticeable difference. However, your SanDisk Extreme Pro card is pretty fast, so I wouldn't think that your LaCie would be any better.
Some thing you might try are:
Shut down any CPU intensive programs that you might have running while importing to iMovie.
I don't know whether this would make a difference but you could try using the Image Capture app on your Mac to import directly to your Mac rather than into iMovie.
You could also try importing into the Photos app and then accessing your videos from the Photos item in the iMovie project sidebar.
I don't know whether DropBox would be faster than your SD card. I would suspect not.
-- Rich