Lightroom is an application that just performs functions on files that exist. The card reading, copying, and recognition is based on hardware and the OS's implementation of the driver and file system. LR has nothing to do with bad card performance. It'll be the card reader/driver, OS ExFAT implmentation, or card itself.
Usually, a slow XC read (or HC for that matter) is because the card reader isn't DMA compatible with the box it's in. It can also be slow because it's actually a USB bus reader (even if mounted internally). The only real way to get full speed out of a high speed card that's also high capacity is to use a USB3 or FW reader. An XC card is mostly the same as an HC card, but formatted with ExFAT instead of FAT32 so that the full capacity can be allocated.
On top of that, most internal implementations of card readers are for convenience, not speed, so they never reach full capabilities of the media anyways. Try shoving that card into another reader (if you know somebody who has a USB3 reader, FW800 reader, or SATA-SDxC reader and see if your speed improves.
Also, I've had to recover many a PNY card for clients - they seem to be ok at first, but not so stable after some usage. They're not a "cheap" no-name card, but they aren't the best manufacturer either - I won't use their "Pewny" memory in any box and I don't trust their media cards for my cameras.
If it's the ExFAT format on the card, you could always try formatting it in the camera, then shoot a few frames, and see if reads any better.
I have 3 Macs, but don't use any of them for LR, imaging work, or even dumping cards - they're pretty much music players or paper weights. I keep them because I have to do compatibility testing and some file conversion to other formats. However, since ExFAT is a Microsoft designed file system, I would also suggest that you try your card on a Win7/8 box with a USB3 reader on a USB3 port and see how the card performs.
If you know somebody with a Win7/8 box and a USB 3 card and reader - run this on the card:http://mympx.org/Downloads/p13_sectionid/2/p13_fileid/13
it'll tell you if it's reading fine. I use it often to check for fake Sandisk cards...