Just want to add another thing:
When editing really large maps with lots of doodads, you will notice a small freeze every 3 seconds (even when doing nothing!). While this does not affect the object editor, it's really annoying when terraining.
To prevent this, use the same method described in the opening post but disable the minimap. This particular lag is caused by the periodical updating of the minimap picture.
You might want to use the minimap to navigate, though; unfortunately, I haven't found a way to prevent the minimap from updating without removing the minimap window as a whole.