The patch size of a compute terrain sometimes messes things up when adding masks after that to terrain displaced before it. If you're able to take it out, you may also win render time. Otherwise reduce patch to 1m or so and see what happens.
Also, in initial terrains, I always (mostly) use a stretched Y to get the same fractal color over height, which is also handy when deriving maps through 'displacement to scalar' for lakes and such.
And Daniil's shader only takes a lot of time for the first render, when memory is filled with data. Later it's faster.