The tessellation itself has no performance impact whatsoever. I was curious about this too before I started and found that other people didn’t have any issues with it. Even without tessellation the rock displacement looks good enough.
One thing that gives me a small frame rate boost is setting the “LODDistance Factor” on my landscape. This controls the distance at which the terrain loses detail, mountains in the distance will start to look chunky. The default value is 1, I currently have it set to 5.