Three things impact real-time in the Multigrid: the number of tracks, whether or not you have cross-fading checked, and of course the computational complexity of the individual Sounds.
If cross-fading (or Audition) is checked, then any pair of Sounds in the track have to be able to be scheduled in parallel. So one thing to try if you get the OORT (out of real time) message is to uncheck cross fading.
If you were to take the most complex Sound from each of the tracks and play them all together in a Mixer, they have to be able to play in real time. So another way to reduce the complexity of a Multigrid is to reduce the number of tracks.
And finally, if there is a single Sound (or combination of Sounds on parallel tracks) that runs out of real time, then it's necessary to open up that Sound and look at how it might be simplified. For example, if it is an analysis/resynthesis, perhaps the number of Oscillators could be reduced.