Well I got curious, and wrote a test program for my Linux RPi: 
Doing the equivalent of what du(1) does (a recursive tree walk statting every file) seemed to be about 15% faster with openat/fstatat than with open/lstat. This was on a local drive (SD card).
Over 9p to my Plan 9 RPi from Linux it appeared to make no measurable difference at all (though I don't know whether v9fs can take advantage of the *at capability).
Maybe there's some use case where it makes a more dramatic difference. My tests were not very scientific.
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