well this is kind of a theoretical issue with a some practical effect. I hope master zapp will take a sneak peek (cause afaik he wrote that shader...)
question (sort of rethoric)
how large are the flakes in a real car pait? hmmm I didn't measure it out, but I guess like 0.2 - 0.5 mm? right? well, lets think the car is 5m long. say, the flake size is sort of 0.5 mm. its easier to count.
I'm aiming to the control slider (or input tab) in the attribute editor adjusting the flake size.
as mr does not use meters or what ever but units, lets say the car is 5 units or even 500 units long (500 it helps a bit, but not that much). what size are the flakes on a 5 units car? a 1/10000 right? oh boy, thats damn small... say the car is 500 units, than the size is 1/100. ok, 1/100 is more usable value but still rather small. it will not change the flake count though.
i think - and thats just a personal impression: the smaller the flakes, the longer needs the shader to render. just turn me right, if thats a b.s.
and hmmm before u start to model, do u really extend the grid over 500 units? well - u should if u want to model a car in real world units.
theres a "fade" control for the flakes, dependant of the camera distance from the model. so the flakes are sampled at a close up and they fade out with the distance. if u shoot a close up, u should see a nice metallic effect. if u shoot from distance u should still see the effect of the flakes, but those arent resolved anymore.
even with the "fade" are all the flakes being computed and if the size is out of the sampling radius, the get ignored? if yes - thats the answer the shader is so slow to render...
a guess: could be the solution doing some "switch"? like from distance X the flakes are computed (the size is higher than the min sampling radius) and above that distance X, they are replaced by some "glossy" shader?
to make it more clear: if u shoot the whole car the flakes arent computed and are completely replaced by the glossy shader. in this case the flakes could get real world size (0.0005) and the count of the flakes will stay in some reasonable range. I mean - u can type that value into that slot yet, but it still feels like a crazy small value - other way - there are trillions and trillions of flakes there.
if u take a look at the teapot in the shader description - its a very tiny teapot or the flakes are like 1 square mm large. (http://www.lamrug.org/resources/doc/images/carpaint-flake-decay-off.jpg).
even in the picture where the "dirt" effect is demonstrated - just take a look (http://www.lamrug.org/resources/doc/images/carpaint-dakar-dirt-wr.jpg).
very large flakes.
i hope u'll get what I'm trying to point out. I appreciate any idea how to handle that thing.