I believe that the people with whom you spoke are referring to getdents() being slow when the cache is cold. That is because ZFS does not at this time implement

I believe that the people with whom you spoke are referring to getdents() being slow when the cache is cold. That is because ZFS does not at this time implement directory prefetch logic. That will change in a future release of ZFSOnLinux, but it is a low priority because cold cache performance is not terribly important for production use. I am beginning to think that not only do you not use ZFS, but that you have never used ZFS. I suggest that you try running your own benchmarks and workloads on it. It is a joy to use. I think you would agree if you were to use it in a blind fold comparison test.