Volume display in the viewport can be an expensive drawing operation. The volume quality can be adjusted to provide better viewport performance or better volume quality.
Values
hou.viewportVolumeQuality.VeryLow
Quick volume preview.
hou.viewportVolumeQuality.Low
Minor quality loss for performance.
hou.viewportVolumeQuality.Normal
Balanced quality/performance setting.
hou.viewportVolumeQuality.High
Jittered, high quality volume with more passes.