I'm guessing it's due to the fact that your machine is only a dual core processor with only integrated graphics not dedicated graphics and so it's taking more system resources than the machine can handle when running video especially if you're outputting at 720 or higher ... If you are using an external for your videos via USB that could be hindering performance as well since the USB bandwidth is shared with each other ... If you do use an external try out a thunderbolt external and it may help to open up that USB bandwidth a little
