Licensee:
"Content creators from my team have reported that when they replace actors in an open world it can take more than 10 seconds. This is with several thousand actors loaded in the world. On my end, it takes 10.2 seconds to complete UEditorEngine::ReplaceActors where most of the work is during the construction of FArchiveReplaceObjectRef. This is serializing a number of objects in the world (several with bulk data).
see images in link.
Individual SerializeBin operations don't take too long, it's just the sheer number of calls needed."
Requires further investigation.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-169553 in the post.