I am importing from a sandisk UHS-1 sd card with a usb-3 internal card reader (max. SD card speed is 95 MB/s - tested!). And LR imports (reads) fast from my card, then it creates the previews. Im running on W8.1, i5 @ 4Ghz. That's a totally diffent hardware than yours, but I do not think LR is the problem.
Try to copy the content of your card to the interal hard drive (using finder) and import ("Add") from there. Check the seed of copying the data and check the speed of importing.
Try diffent USB ports, if available.