As our levels have grown in size (10k+ actors) we are seeing very large hitches caused by World Outliner every time an Actor is added or removed.
These hitches (on the order of 150ms on a newish PC) happen only when the World Outliner is open and displayed. If we select a different tab, (e.g. select Levels) causing the Outliner to not be displayed the hitches go away. Collapsing folders within the World Outliner has no impact on performance but setting a search string that reduces the content shown in the Outliner can improve or completely alleviate the hitches (e.g. if the actor which is being added is not matched by the search string, there will be no hitch).
While we have workarounds for this issue, they’re far from perfect and so we’re looking for a better long-term solution.