So with a USB3 card reader in a USB3 port, you have only a few bottlenecks to be aware of.
The speed of the card - which despite what the young guys think, isn't usually a big problem
Space available on the disks you're writing to, there used to be additional problems with disk fragmentation, but nowadays that's almost always taken care of
So that leaves us with what is LR4 doing on import? One way to see what difference that makes is to do this.
Just using Windows explorer copy the images to the PC, then open LR and import them to your library from there (without moving the files), that should show whether there's an issue with LR or the PC, if it's the PC, there may be other programs taking up your processor time.