Hi, there is a known bug that can exhibit performance issues, and it can persist on your project for a very long time if you don’t fix it rapidly.
Check out the quote at the end of this post.
You should make sure the bug is fixed before proceeding through any further troubleshooting.
This bug appears as soon as you launch Cubase and when you create a new project afterwards. Once the bug is fixed for a particular project you can save it, and it will now be bug-free even when closing Cubase and opening that project again.
But if it goes to red event at 256 then it might be something else. Make sure you don’t have any anti-virus or “enhancers” running in the background (or more generally any program that has no use, just remove them from startup).