@lawnjelly the graphics calculations aren't a concern at all. The intensive bit is the mixing and equalizing of data between cells. It's called about twice per second. (it is a 'passive' fluid simulation for a game, though typically gasses as opposed to liquids.)
Yes, i know what it is. I'm just not sure how to quickly and efficiently do it on a potentially large scale.
I've explained that it's for fluid simulation, typically of gasses. Here is an example of what you might see:
21 moles of oxygen, 80 moles of nitrogen, at 20 degrees celsius in 'group A'
100 moles of carbon dioxide, at 1000 degrees celsius, in 'group B'
Not only are we spreading the molar count evenly between the tiles, but also the temperature (which affects the pressure of the fluid, and thus the rate at which it disperses)