Skeletalmesh editing can be slow when asset have a lot of morphtarget

After investigation, the asset is not rebuild, what took time is the 2 following part

1. The serialization of the DDC where we have to initialize the morph targets
2. E:\P4\main\Engine\Source\Runtime\Engine\Private\SkeletalMeshLODRenderData.cpp DivideAndConquerPermuations function use in the initResources of the render data

I use a GDC demo mesh with ~3100 morph target and it take

1. Applying the DDC cache: 40 seconds 
2. creation of render data (InitResources): 65 seconds

I'll investigate on how I can improve the performance of those 2 parts

Import a skeletalmesh containing a lot of morph target

change UMaterial point by one of the material slot.

RESULT: It take some time to change the UMaterialInstance because the skeletalmesh get rebuild.

EXPECTED: The skeletalmesh should not be rebuild when we change the UMaterialInstance.