Well, my "particles only" idea seems to be wrong - cells in lookup table needs to be smaller than one particle. Huge count of small particles wants a huge lookup table too.
Definitively there are a lot of parameters, able to slowdown the render. Lookup table, marching settings, evaluated dept, alpha, ray-marching quality...
Nothing new when it comes to ray-marching render, anyway.
This one is rendered with a help of nodes from BA plugin - exactly, there is a 'mesh distance' node which makes a smooth falloff, when volume shader is applied to mesh. Theoretically, something similar could be done by filling a mesh volume by particles - but I'm not sure the result is same...
No any particle for this, just a mix of animated fractals. Render time for one frame is about 40 seconds on quad core, using 3 lights with shadows, AA 0-0.
There is small camera movement too. Beside default settings, a bit of 'color absorption', stronger shadow density.
FLV with animation