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The material I have excerpted below is in fair use quoting from the author and the copyright proviso.
“Should I use High efficiency raw on the Z9?”
My short answer is currently: no. The tendency of everyone is that they want to use High efficiency* on the Z9 because it makes the buffer less limiting. Of course, with the right card, I don’t find the buffer limiting with Lossless compressed.
The problem with the High Efficiency settings is simple: Nikon hasn’t done anything—other than putting in basic support in NX Studio—to make them useful to photographers. Meanwhile, IntoPix, whose intellectual property underlies the High efficiency options, isn’t doing anything to give converter makers useful information, at least not that I can see. So we’re currently in a situation where I’d say we don’t have optimal raw conversion capabilities for High efficiency. Many converters don’t support it at all.
I suggest to any reader interested in the full commentary that one reads the public web site referenced above. I personally use Delkin Black CFexpress cards and have no Z9 buffer issues, including “short” bursts of CH (short is a “few” seconds).