Licensee says that adding multiple InstancedStaticMeshComponent instances can be very slow (main bottleneck being the physics building).
I think there's a good case for a method something like:
void AddInstances(const TArray<FTransform>& InstanceTransforms, TArray<int32>* OutOptionalIndices);
TArray<int32> AddInstances(const TArray<FTransform>& InstanceTransforms, bool bReturnIndices);
which adds multiple instances at once (preferably even in parallel), and performs physics building asynchronously, in a similar manner to ProceduralMeshComponent.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-82372 in the post.