Windows is a multi task operating system. When your games are running, other programs and services are also using resources.
Processing time is shared between programs, and the way the time is shared is made by a scheduler. Some process have higher priority than other process. This priority setting affect how often the scheduler will give CPU execution time to a program.
Some performances inconstancy has been measured with Annwvyn engine apps (and in general, other VR games) on “normal” priority. This is probably due to the fact that, unlike “normal” games that you generally use on exclusive full-screen or with a front focus window, VR applications are pushing they rendered images to another (small) program on the VR runtime. This may cause windows to think that you're not actually needed as much resources as you need.
This can be overridden by setting the priority to high. By default Annwvyn does it on the AnnEngine constructor. You can prevent this by puting the static bool AnnEngine::autosetProcessPriorityHigh to false before calling AnnInit().
AnnEngine also has methods that permit you to switch between “normal” and “high”. They are also accessible via the ChaiScript interface