I'm not really the ZFS pro, I only compile the packages for each Unraid version so that they are available for everyone, I'm only using ZFS for my Cache and and for my appdata and VMs without any snapshots whatsoever because I'm not really a big fan of ZFS anyways because it's the complete opposite of that what Unraid stands for and is not really my use case except for the "Cache".
The right persons to ask are @steini84, @BVD, @Iker,...
But from what I see your read/write speeds are really slow, there must be a bottleneck somewhere, on my NVME drives I reach almost the specified speeds of about 2.500MB/s.