Want to check on this again. This continues to be probably our largest problem with UE4 in our workflow, and here we are now many many versions later still dealing with the same problem. Is there any chance this has been looked at at all for 4.15? This isn’t exactly and edge case situation, and can easily be seen by any artist for example using HISMs to generate large numbers of instances. Just selecting the HISM with a few thousand instances can freeze the editor for a minute, and then the UI time spent drawing all those controls that aren’t even visible has a significant impact on frame time.
Getting Slate to not waste cycles drawing hidden UI elements would be a huge win for everyone in terms of editor performance.