Hyper-Threading and ASIO Guard
Hyper-Threading has been introduced back in 2002 with Intel's Xeon and Pentium 4 processors. Many of today's Intel-based systems on both Windows and Mac OS X utilize Hyper-Threading to increase performance by a more efficient usage of CPU resources. Details on Hyper-Threading can be found in this Wikipedia article.
However, in the past we have found that having Hyper-Threading (Hyper-Threading) enabled can lead to performance issues and spikes in the VST performance meter. At very low latencies, even audio dropouts may occur. The reason for this is that other Hyper-Threading processes on the same physical CPU might interfere with real-time audio threads on which our software, unlike many other applications, is highly dependant.
We usually recommended to turn off Hyper-Threading whenever performance issues occur. If applicable, this recommendation is still valid for sequencer versions older than Cubase 7 or Nuendo 6 (most Windows systems offer this option in the BIOS or UEFI setup, whereas Hyper-Threading cannot be disabled under Mac OS X).
With Cubase 7 and Nuendo 6 we've introduced ASIO Guard. Depending on the project setup, this feature reduces the real-time audio processing load significantly and thus reduces the risk of audio dropouts caused by Hyper-Threading. For details on ASIO Guard, please refer to this article.
Enabling Hyper-Threading while ASIO Guard is active usually has a positive effect on the overall system performance and is the recommended combination. 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.