Why do I get choppy playback in Adobe Premiere Pro cs6?

The playback is really choppy.

What I am using:

(Brand New) iMac

Processor  3.2 GHz Intel Core i5

Memory  16 GB 1600 MHz DDR3

Graphics  NVIDIA GeForce GTX 675MX 1024 MB

Nikon D5100 DSLR

Shot in 1080i 30 fps

Thanks