I am afraid that we have the same problem: ~90 Rocky8 VMs migrated from VMware, pegging one CPU very often. We have suspended further migration to XCP-ng due to this issue.
Has been the root cause identified since 2024? Is there a solution or workaround (apart from upgrading to Rocky9)?

@florent Thanks for your reply! We have started to migrate thousands of VMs, so disk transfer speed is important for us.. We will also do our detailed tests soon with different threads setting and publish it here.
I think threads=1 is a good and logical default, but not efficient. Others might complain if you set it to a higher value. Configuration option would be a real good solution.

Is there any specific reason to limit nbdkit server to threads=1?
spanwNbdKitProcess
According to our tests significant improvement (up to 4 times) could be achieved when increasing thread count.
Is there any plan to make it an user configurable parameter (in xo-cli for example)