Blender 2.78 (2.78 2016-10-24, Blender Foundation)
Mac Book Pro Retina 2013
2.6 Ghz Intel Core i7 - 16GO 1600 MHz DDR3
In my scene with a parent DupliVerts object (Properties /Object / Duplication / Verts), with inside of if instances of Group.
When those childs instances are from one group, that's ok.
But when they are from several groups, the 3D viewport render preview is stucked to the sample 1, rendered without ending !
Good for heating the room ;)
To test, simply [Shift] [Z] in 3d the viewport in my blend to see this issue
Hope this help, keep the good work !