Performance drop in Orthographic views when Visualize->Volumetric Lightmap

Severe performance drop when when entering Orthographic view, but only when Volumetric Lightmap is selected.

1. Create a new level (default, with sky sphere etc)
2. Run Stat FPS
3. Add LightmassImportanceVolume of 5x5x5
4. In world settings, change "Volumetric Lightmap Detail Cell Size" to 20
5. Build lighting
6. Show Volumetric Lightmap by selecting Visualize->Volumetric Lightmap
7. Go to any Othographic view (e.g. Front)

Results: The framerate drops from 120FPS to about 5FPS.

Expected: The framerate to continue running in normal ranges.