I read somewhere that you can hold down SHIFT and the flattening tool to make the flattening process usable. I find this technique actually works. So I am unsure, if this is what you're speaking about.

i recently did it with a hoe. just a dirt road from my Farm house to my Coal factory, but it was easy

i ment perfomance wise, does it still cause perfomance issues when you flatten large areas?

Excessive terrain manipulation still causes performance loss as far as I'm aware.

thank you!