In-editor hitch when deleting one actor in a world with many loaded actors

When editing a level with many actors (1000+), deleting an actor results in a noticeable hitch: the editor becomes unresponsive for seconds. This hitch becomes longer the more actors are loaded and exceed 10+ seconds on maps with tens of thousands of actors. This impacts productivity on projects which commonly have large worlds with many actors.

Most of that time is spent in 

FBlueprintEditorUtils::GetActorReferenceMap() 

which is called in

UUnrealEdEngine::DeleteActors() 

to detect references to the to-be-deleted actor and present a warning popup:

Actor {0} is referenced by the level blueprint and other Actors/Objects. Do you really want to delete it? This will break references. 

Temp workaround

As a temporary workaround, the hitch can be avoided by disabling the warning functionality by opening Editor Preferences and disabling bCheckReferencesOnDelete, or directly in DefaultEditor.ini or Saved/Config/EditorPerProjectUserSettings.ini:

[/Script/UnrealEd.LevelEditorMiscSettings]
bCheckReferencesOnDelete=False 

Goal

However, as the warning for existing reference to the actor is quite useful, it would be better to investigate ways to optimize actor deletion that don't result in a seconds-long editor hitch for large maps while retaining the existing references warning.

• Create a level with 10.000+ actors
• Save the level
• Delete an actor by select one and pressing Delete
• Observe that the editor freezes for multiple seconds