Frame stutters - what are the causes?

[Insuber]

Oleg insisted to model the exact trajectory of each smoke particle, including Brownian motion and integration of the Navier-Stokes equations. This gave a hard time even to the 11000 Petaflops Riken supercomputers taken by beta-testers as the average simmers' rig. Oleg was seen leaving the building mumbling about two more weeks for the optimization of the Navier Stokes integration.

Et voila what gives your stutter today.