Bob
Can you be more specific? What do you mean MPEG? The formats that work best with QLab 3 are ProRes 422 LT, PhotoJPG, and H.264 for videos without transparency and ProRes 4444 for videos with transparency.
As an added bonus, ProRes looks a hell of a lot better than MPEG 2 and even somewhat better than MPEG 4, in my opinion.
Some other formats work, but not as well, and still others do not work at all even though they might work in QuickTime Player. This is because QLab 3 uses the AVFoundation framework to play video, which is the framework Apple has blessed as the way of the future, whereas QuickTime Player uses the QuickTime framework which is compatible with a broader range of formats but is also fraught with issues dating back to the early 90s.
I also second the commentary that a MacBook Air is probably not the right Mac for any heavyweight video work. The GPU isn’t particularly powerful, and perhaps more important there is no dedicated VRAM; system RAM is shared between the CPU and GPU. This is also true on many models of MacBook and the Mac Mini.
If you’re going to use a non-retina MacBook or a Mac Mini for video, you want to make sure you have at least 8 GB of RAM installed for the computer to make its best effort.
Cheerio
Sam