https://www.dropbox.com/s/7p1torz8l18p5o6/rawsandjpegs%20500ms.zip
Thanks for testing. It looks like this camera needs the longer pause in some cases. It would be unfortunate if this caused a 500ms delay in normal shooting, so I've made a new build and version of the fasthdr script to test this.
This version of fasthdr will record a log file called fasthdr.log in the root of your SD card. If you can run it and post the result log, that will be very helpful.
Additionally, if you go to "miscellaneous stuff" -> "Debug parameters" and enable "misc debug values" in the chdk menu, it will show the last wait time after FP: and the max wait time after MP:
If you can shoot a few shots manually with raw enabled and report the values, that would be helpful. Note max value is only reset on reboot, so this should be done before running the script, or after a fresh boot.
Regarding the DNG exif issue:
The "fasthdr" script above and the filecounter change are not expected to resolve this. The conthdr.lua script above probably would, but it requires CHDK 1.3, and I haven't checked in the filecounter change yet, so you probably get the missing DNGs again.
You can probably also avoid it in the old fasthdr by adding a small wait after the exposure count changes. I've added an option for this to the version of fasthdr attached. Set the "exif wait" parameter to something like 10 or 20 or 30. I suspect 10 will do the trick.
Thanks again for bringing this up and testing. Since we don't have every camera, it's the only way we can get things like this fixed.