It could be any number of factors that is causing frames to drop (e.g. large keyframe distances, high bitrate videos, CPU usage, etc). Playing 720p videos is quite a huge step up from the usual 640x360 videos that people play.... though your computer should be able to handle it, and the 2011 MacBook Pros definitely have no problem playing back multiple 1080p streams. Are you getting any keyframe warnings for the videos?
