Cloth painter hangs while pressing H to preview the masks

The DerivedDataCache for the whole SkeletalMesh doesn't need to be rebuilt while previewing the mask painting, since it will be rebuilt when exiting the painter anyway. 

1. Build and run EngineTest.
2. Open the Owen Skeletal Mesh asset in the mesh editor.
3. Click on Activate Cloth Painting.
4. Select the OwenCoat cloth and the Max Distance mask to paint.
5. Press the H key to preview. 
6. Observe the very long pause while the DDC is being rebuilt.

(Note it was always freezing a bit, but that used to be a lot more reactive)