Adobe premiere is lagging in preview window
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).