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When the world settings window has been opened but is not visible, the destructor for FLightmapCustomNodeBuilder is never called to remove the editor delegates. Opening a large number of maps has been reported to cause performance problems due to the ever increasing number of delegates. See repro steps for further info.
Open the World Settings Window and then switch to another window that is docked with it so that it is no longer visible.
Place breakpoints in FLightmapCustomNodeBuilder::SetOnRebuildChildren and FLightmapCustomNodeBuilder::~FLightmapCustomNodeBuilder, which add and remove the delegates.
If you keep opening new maps you will see the breakpoint in SetOnRebuildChildren will get hit but not the second. The destructor only gets called when the editor exits.
If you repeat this with the world settings window visible the delegates get deleted correctly.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-25725 in the post.