Quote form our resident shader guy Chetan in response to your question:
Quote
This version of godrays improves perf by avoid the need to re-render the entire scene for masrking etc. This is one benefit of our new gbuffer passes that we have all the buffers (depth/normal/misc) needed for all the lighting and post processing needs. This change is the big improvement over previous godrays. Gbuffer pass itself is upto 2x-4x faster than normal shading pass depending on quality modes and other factors.
There's also a huge quality improvement compared to previous version and we have enabled them for new local point lights.
Second improvement comes as a part of new godrays implementation itself. There's a minor improvement in something called fillrate due to different ways of doing things. One big change is we are now performing radial blur pass on a B&W texture instead of RGB. But again fillrate improvements might not be noticable depending on GPU/Resolution etc because at the moment godrays is being done of full size render and number of blur passes are high. Consider this as INSANE quality mode for godrays. We didn't got time to implement quality modes for godrays for beta update.
To be honest it was least on the priority list since we didn't notice major perf impact during our internal testing (we mostly tested stuff on 1080p or 2140p sometimes). Also, we are planning to do another overhual of godrays (no promises). I wanted to try one of the screenspace ray-trace (or ray march) based implementation which is the standard now on dx11. But its tricky to implement on dx9 and may or mayn't turnout to be faster that current radial blur based implementation. Apart from better perf it will improve the quality even further and might enable us to use them for new local spotlights the way we wanted. (But again no promises, it's something on the list for xen release but not big priority at the moment.)
Another areas of improvements include - making godrays go through tonemapping pipeline, improve AA when MSAA is enabled , etc
)
