I have many video file on my mac with various file types (.mov .avi .mp4 etc.). My questions is why do some files seem to make the computer lag? In other words when using quicktime, I have two files.. file 1) .mov- codec:H.264, AAC, Timecode / 1920 × 816 - 24fps file 2) .m4v- codec:H.264, AAC, 1280 x 718 - 60fps File one runs a lot smoother on quicktime.. especially when dragging the slider to certain parts. It's larger both in file size and resolution.. File 2 fetches up a lot. I am a novice with stuff like this and just wonder what factors cause issues like this.