A "rocky border to the world": Needing a solution that wont compromise performace

In my game you play on several "shards" of the world that are floating on a sea of Aether. Well, each needs a sort of "rocky" edge that makes it seem like they were torn from the world's crust. I have tried enlarging regular rock models to accomplish it, and although it looks nice, it hurts performance to bad. Any solutions?

 

It was made for this purpose exactly: https://assetstore.unity.com/packages/tools/utilities/amplify-impostors-beta-119877

 

Or here's a free alternative - https://github.com/xraxra/IMP