Hi Mike, we have an open Veeam case 02060809 for these issues but contacting MS Support is terrible as mkretzer can confirm so we haven't tried yet,,, (feedback is just not to use ReFS en stick with NTFS.... why release it and promote it)
The Veeam case was created to know when we could install the 'horrible' KB on our repo servers so we can continue ReFS performance troubleshooting --> now were just waiting for any progress either from MS or Veeam
""Veeam support engineer:
Concerning the ReFS performance - I am afraid I do not have any information about that either. We are waiting for any update too, as we have many cases related to performance on ReFS repositories no solid solution yet unfortunately.
We have also some information about abnormally large fragmentation on ReFS volumes used with Veeam and it might be related to the issue, but no solid information yet.
Also according to Gostev sometimes storage spaces (in S2D) configured improve performance and merge reliability for ReFS repositories, however it is not the configuration I would advise as a first step.