ZFS does suffer from read-modify-write on partial record writes. The effect of that is apparent in the benchmarks. However, the benchmarks are being done on mechanical disks, which have low IOPS. The IOPS of a mechanical disk are roughly the same on a given sequence of IOs at different positions regardless of whether they are 4KB or 128KB in size, so it only has to pay a penalty once. If the record size were changed to 4KB, this penalty would disappear and ZFS performance should increase, provided that the VM internals are properly aligned.

Also, read-modify-write overhead reduces IOPS by at most 2 and bandwidth to the smaller of the link bandwidth and the IOPS times the record size. A CoW filesystem should be able to perform roughly at that level when it does read-modify-write on records/extents. Unless btrfs' internal extents are huge, there is an issue somewhere. Of course, having huge extents by default on which read-modify-write is done could also be considered a design issue.