Maya.mp4 -

If you use an MP4 on an image plane, it may drastically slow down your viewport. If it does, switch the image plane's display mode to None when you aren't actively using it as a reference. codec issues - Forums, Autodesk

If your video won't load, it is likely a missing system-level codec rather than a Maya bug. Maya.mp4

If you need to import an MP4 into Maya to use as a background for tracing movements: If you use an MP4 on an image