Simple answer: don’t make them.
With a populated editor window Unity3D becomes unresponsive and laggy, just like all the ‘we-offer-the-same-GUI-on-all-platforms’ apps (Blender too). The terrain system in Unity is still in 2007-2008, even free engines offer better tools today. Of course you can spend money in the Asset Store and if you are lucky, the utilities you buy will work properly for making rivers, roads, scattering and so on. But it still will be very-very slow.