After investigation, the asset is not rebuild, what took time is the 2 following part
I use a GDC demo mesh with ~3100 morph target and it take
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.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-89937 in the post.