I just re-tested firmware v1.10 - and my findings were wrong initially. Please watch through for the accurate story.
TLDR - the Atomos recorder slows down performance and doesn't reflect what you will get in the real world.
See my CF Express Guide test results here: https://learn.mattgranger.com/courses/CFXB
Expert Nikon Set up Guide here: https://learn.mattgranger.com/courses/zsetup
Cards used 1 - https://bhpho.to/3GMe5ur