I have a mac as well, and I'm thinking it's more than likely not your computer but your internet connection. Otherwise, my other thought would be that your not saving it at a low enough bandwidth when you save the video. I've had that problem before. When you go to export the video to put it in save mode make sure it's not more than like 340 kbpps (i'm not sure i spelled that right i'm not good with all the technical terms). but there should be a spot to save it as that size. If I don't make any sense I apologize but i'm hoping you'll see what i mean when you go try to re-save the video.