MMCSS
The Windows service that gives audio and video threads priority with the scheduler, so everything else can't crowd them out.
The Multimedia Class Scheduler Service arrived with Windows Vista in 2007. An application calls AvSetMmThreadCharacteristics with a task class such as Pro Audio or Audio, and the service raises the thread's priority so the audio callback runs ahead of indexing and updates. To avoid starving the rest of the system, 20 percent of CPU time is reserved for lower-priority threads by default.
Vista's first release had a well-publicised side effect: to protect playback, MMCSS throttled network traffic while audio was playing, and users on fast networks found file copies slowing down the moment they put music on. The NetworkThrottlingIndex setting has been a fixture of every Windows audio tuning guide ever since.