I think the story with multiple cores, asio guard and cubase still has not seen a definitive endpoint in it's development.
For more info on the subject:https://www.steinberg.net/en/support/kn ... guard.html
There are a lot of people who experience trouble with modern systems when trying to use the full core power of a system. Using hyperthreading can be an advantage in modern systems, but it can decrease performance too in some conditions. It depends on the way the system is build and the way the system uses cubase and what version of cubase you are using.
To kill at least one faulty idea. HT gives you a double amount of "virtual" cores, but using HT does not at all mean you "double" the resources for your system. HT is a method to try to process things more smoothly. In most cases the differences are expressed in percentages. Read the article and the references in it, and you will be probably a lot wiser. So in some cases your performance can indeed be better with HT off. (only using "real" cores)
Enabling Hyper-Threading while ASIO Guard is active usually has a positive effect on the overall system performance and is the recommended combination for cubase 7. Only in rare cases - e.g. with projects using many "live" tracks - Hyper-Threading can still lead to performance issues. Again, please refer to the dedicated ASIO Guard article for details.
To those rare cases we at the forum might add also specific GUI related issues and the fact that some people still have spikes on the asio meter even with the lates asio guard implementation. Other weirdness includes flickering cursors and so on...
It looks like the graphics handling and the asio handling is done by the same "real core" in the OP's computer which creates some sort of traffic-jam on that core. DIsabling HT gives better performance for him.
But again: it is just my opinion on this, and i certainly would like to be given further insight by someone else with more knowledge on the subject regarding this issue on how modern systems with multiple cores should ideally be tweaked to run the DAW's in an optimized way.