Some users report slowing down synthetic fulls or other fast cloning related operations on ReFS filesystems after installing the latest Windows Server 2016 cummulative update in May 2018. The factor can be 3-10x.
This is obviously related to a new version of the ReFS driver that is included in the 05-2018 update. Microsoft ist aware of the problem and is currently working on it. It could be in conjunction with other patches that address Spectre/Meltdown vulnerabilities but currently there is no other workaround to get higher perfomance than either uninstall 05-2018 CU or replace the ReFS driver with the version from February. The "fast" version from February is 10.0.14393.2097, the new and probably slower version is 10.0.14393.2248.