Nvidia's website says: The Global Performance Cache feature makes After Effects faster and more responsive than ever before by taking full advantage of the power of your computer's hardware. NVIDIA GPUs allow you to accelerate previews when drawing images to the screen for a highly interactive experience.
does the global performance automatically use the gpu or do you have to enable it? and is this a good reason to invest in a good gpu or not really?
How much ray-traced rendering do you intend to do?
The global peformance cache has nothing whatsoever to do with the GPU or Nvidia. See this for details of the cache features:
After Effects uses the GPU for almost nothing. Details are here: