Core Image Volumetric Rendering
I'd love to be able to take credit for this shader, but the hard work isn't mine! This volumetric nebula shader code originates from glslsandbox.com . All I've done is changed main() to a kernel function, swapped out gl_FragCoord for destCoord and wrapped the code in a CIColorKernel.
On my iPad Pro, I get around 30fps rendering at 2732 x 2048 - not too bad!