In vtkMRMLModelSliceDisplayableManager::vtkInternal::UpdateDisplayNodePipeline() the polydata gets invalidated without any condition (by calling polyData->Modified()). This causes about 2x slowdown in the slice intersection rendering.

Currently the locator in the cutter is rebuilt every single time a slice intersection is redrawn, while it should be rebuilt only when the polydata actually changes.

There is a comment before the polyData->Modified() call: "need this to update bounds of the locator, to avoid crash in the cutter". If the locator really cannot detect the polydata bound modification properly then we should check the bound and if (and only if) the bound has changed then call polyData->Modified().

How to reproduce:

• Load MRHead example
• Go to editor module, threshold with the default value, create a model
• Enable "Slice intersections visible" for the generated model
• Open the Performance test module and run the Reslicing test
  Rendering time:
  Current version: 1131ms per frame
  After removing polyData->Modified(): 670ms per frame

Tested latest trunk version, on Win7 64-bit.