Use BufferedGeometry

[timkurvers]

http://threejs.org/docs/#Reference/Core/BufferGeometry:

This class is an efficient alternative to Geometry, because it stores all data, including vertex positions, face indices, normals, colors, UVs, and custom attributes within buffers; this reduces the cost of passing all this data to the GPU. This also makes BufferGeometry harder to work with than Geometry; rather than accessing position data as Vector3 objects, color data as Color objects, and so on, you have to access the raw data from the appropriate attribute buffer. This makes BufferGeometry best-suited for static objects where you don't need to manipulate the geometry much after instantiating it.

[timkurvers]

In use for terrain since 1fed3f3, and it is blazingly fast.