Simultaneous multithreading (SMT) or as Intel calls it "Hyperthreading" 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 Hyperthreading to increase performance by a more efficient usage of CPU resources. AMD has also implemented this feature in their current "Zen" processor generation (Ryzen, Epyc). Details on simultaneous multithreading can be found in this Wikipedia article.
However, in the past we have found that having simultaneous multithreading 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 SMT processes on the same physical CPU might interfere with real-time audio threads on which our software, unlike many other applications, is highly dependent.
We usually recommended to turn off simultaneous multithreading ("Hyperthreading") 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 "Hyperthreading" 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 simultaneous multithreading. For details on ASIO Guard, please refer to this article.
Enabling SMT/"Hyperthreading" 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 - SMT can still lead to performance issues. Again, please refer to the dedicated ASIO Guard article for details.