If you have dense sections of doodads like e.g. a forest.
You can always take out the middle sections as players wont be able to see them anyways. So you're left with the borders
The lag problem only really applies at the beginning, where the doodads are being loaded and rendered, and when there are lots of them ON-SCREEN at once.
You could fill a corner of the map with fires and lightning bolts until merely looking at it made your computer freeze, but if you didn't look there, it wouldn't cause a problem.