You must do this on a desktop. While playing hit ~ to summon the console window and use the WidgetReflector command. From there use the Pick mode to select a widget in your hierarchy. Use ESC to stop selecting. You will get a sense for what widgets slate is currently processing.
Only binding to blueprint functions is slow. You can bind to C++ or to properties. But if you have a blueprint function in there, it will execute every frame and might get pretty slow.