rendering time

For 10" at 300 dpi, you're rendering 9,000,000 pixels, or 1250 pixels per second if that takes 2 hours.

For 20" at 300 dpi, you're rendering 36,000,000 pixels, or 8 hours at the same rate.

40" would take 32 hours, it's an exponential scale, not a linear one.

I can't say why a 40 inch square rendering never finishes, but maybe you're just not giving it enough time. If you want to increase the rendering speed you need a faster processor, not more memory.

Wow.. what filter is that?

Some filters can be optimized dramatically (not all though, the look in some filters has its price..).

Here is the list of top performance killer elements:

1. Ambient Occlusion
2. Bitmap based components (blur, motion blur, high pass, sharpen, minimum /maximum, median, percentile)*
3. Bomber
4. Refraction*
5. Derivative*

* performance is related to their position in the overall filter tree..

But often there is no simple replacement, and you have to analyze the construction carefully..

Sometimes you can simply bypass the bitmap based ones and see if the change in final look is acceptable (the simplest way to try that out is to set their radius to zero, then they internally bypass).

I think I posted an optimized version of emme's sketchy painting once, but there is something wrong with the forum currently (you can't see the comments for the filter)..