Adobe premiere is lagging in preview window Video in source preview is lagging but it plays fine in video players. What may cause this? Why premiere is not using standard codecs for source preview?

Specs:
Video is: h264 2704x1536 @29.97fps (GoPro footage)
System is: Core i7 2.3, 8GB RAM, GeForce GT 650M (MacBook Pro, bootcamp)
Adobe Premiere CS 6
Premiere's preview isn't simply a viewer and can't make as much use of caching as it needs to be able to scrub quickly in any direction.  Interframe compression also trips it up for the same reason.  I believe it also processes the entire amount of data.  The video size you are using is pretty large and bandwidth intensive, so unless you are working from very fast disks (solid state) (or a fast CPU if it is heavily compressed) and have fast RAM to deal with the rate of uncompressed data, it is easy for the playback to fall behind.

The related questions talk a bit more about methods of getting around this (such as letting Premiere finish generating conformed versions of the video or reducing the resolution of playback so that not all pixels have to be rendered).