The WaveRT port driver has performance advantages over both the WavePci and the WaveCyclic port drivers. WaveRT avoids any need for intervention by the miniport driver in the flow of audio data between the application and the audio hardware. This feature is particularly important for clients running Windows Vista that use real-time threads to reduce audio-stream latencies. In contrast, while streaming audio data, a WaveCyclic miniport driver must copy data, and a WavePci miniport driver must continually acquire and release mappings that contain audio data.
http://www.microsoft.com/whdc/device/audio/wavertport.mspx