I think you hit the nail on the head, Dave. Thanks.
I am using a SanDisk Extreme 60MB/s 32GB CF and a SanDisk Ultra 20MB/s 32GB SDHC card. When I use the CF as primary and the SDHC as overflow, the green light = monitor blank time goes to almost nothing. If I switch primary and secondary, the write time is almost, but not quite, as long as my earlier setting. All of this makes sense.
Originally, I was running with CF as primary and SDHC as backup. This seems to mean that they are both written to in serial fashion. Even with CF RAW and SDHC JPEG the write time is significant.
So I'm wondering what other folks do with the two card slots. I figured it was good to have a backup, but then again I never lost an image still in the camera with my D700, so do I really need a backup?