Thank you Dan your solution works great!
I forgot to do it! Thank you for reminding me.
The import of sub-components is now fast even if the “Outline” window is open.
One small inconvenience is that the start_operation method can not undo an existing built-in tool selection in my method.
Example:
mod = Sketchup.active_model
mod.start_operation('selecttool', true)
mod.select_tool(DCInteractTool.new($dc_observers))
mod.commit_operation
This is a detail is without much consequence.