To be fair, the Zaxwerks Flag plugin can give good results for certains types of animations – try parenting a small banner to the motion-tracked movement of somebody waving their arms in the air, for example, and the material will flap back and forward in the air in a much more realistic way than other plugins I’ve played with.
But, as the original poster points out, it’s horribly unstable at high resolutions. I just about managed to coax some 4K renders out of it for a project by tweaking the render settings until it stopped crashing, but it was still very slow. For a very high-resolution render for a video wall, I would look at either one of the particle plug-ins, like Form or Stardust, for a stylised flag, or maybe Freeform Pro, if you need something more realistic-looking.
As Dave mentioned above, I would find out whether you actually need to deliver content at the final resolution, or whether a half-res, or even quarter-res render would be acceptable which would speed up the render times. If the display is a low-density pitch LED wall on the other side of a stadium, sometimes full-res renders are not necessary (or can be upscaled after rendering).
Matthew Keane
Freelancer based in Paris, France
– Motion Graphics, Video Editing & Effects, Watchout Programming & Live Operation.