Creating landscape from json file?

I noticed the site is not using gltf to load landscape to display mountain but looks like using json to generate low-poly landscape?

Is it common to load 3D landscape as json and generate on the fly? or I should create landscape in blender and export as gltf? I want to find out which way is faster to display on the web.

An optimized glTF file will be more efficient than the approach shown here.

A JSON file is whatever data you put into it. If you want to compute something custom offline and give it to your app, JSON is a totally reasonable way to do it. But for large datasets (like mesh vertices) a binary file will be more efficient than JSON, and glTF takes advantage of that. Ultimately three.js will have to upload binary data — not JSON — to the GPU through WebGL.