Editing and playing are different animals. Your MBP is woefully inadequate for editing such material, but will play fine. If you want to edit such material, you need a much sturdier computer with a lot more HP under the hood.
A snail can play, but you need a cheetah for editing.
Standard Media Players as Harm mentions have very streamlined playback buffers and caching since the player just reads media with no other function. Editors are advanced players that have far different playback buffers and caching due to the realtime editing capability. This means they require far more ram and also use more processing for memory management. All of this has to be done in the same segment of time a standard player utilizes those frames. Add on this the API for the interfacing with the application which is far more extensive than a player such as Quicktime along with GPU/driver interfacing and you have far more data to process in a segment of time than just playing back a media file in a player only. BTW Premiere currently realtime encodes the original media to the Adobe I-Frame codec for preview which is a significant amount of this processing.
Increase your ram to 8GB bare minimum and look for faster HDD solutions. That would be the place to start.