Comment to RimmyD’s answer.
I turn off “Update BSP Automatically” and “Build geometry” from building menu (in UE4.3.0).
See my “Output Log” after moving BSP-box and “Build geometry” (it take 15 seconds):
MapCheck: New page: Map Check
Cmd: MAP REBUILD ALLVISIBLE
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_42) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_10) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_11) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_12) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_13) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_15) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_17) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_3) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_128) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_0) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_1) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_2) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_24) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_26) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_28) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_29) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_33) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_40) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_43) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_44) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_45) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_46) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_47) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_48) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_49) Not registered. Aborting.
LogActorComponent: UnregisterComponent: (/Game/Maps/Example_Map.Example_Map:PersistentLevel.Landscape1.SplineMeshComponent_50) Not registered. Aborting.
LogRenderer: Adding SpeedTree wind for static mesh Broadleaf_Desktop
LogSkeletalMesh: UPhysicsAssetInstance::InitInstance : Could not find root physics body: CharacterMesh0
Cmd: MAP CHECK DONTDISPLAYDIALOG
MapCheck: New page: Example_Map - Jul 17, 2014, 7:15:18 PM
MapCheck: Info ========== Map Check: 0 Error(s), 0 Warning(s), [111.537ms] ==========
LogEditorBuildUtils: Build time 0:00:15
Cmd: UpdateLandscapeEditorData