Your issue clearly has to do with how Cubase handles “modern” multicore processors. This is discussed in this recent topic, which is now conveniently disappearing in the general barrage in this forum (although a quick browse through the most current postings seems to suggest quite a few issues that may be of this nature, indicating that this is a widespread issue that really needs to be addressed asap by Steinberg - not surprising, since more and more users have later gen Intel or Amd cpus now):
This issue also seems to apply to the latest multicore Apple computers, see here: