Optimized vertex manipulation

Hi. I need to change position of few vertices in my mesh and I need to do it often. However when I try to change vertices position like this:

 _objectMeshFilter = GetComponent<MeshFilter>().mesh;
  _objectMeshFilter.vertices[i] = new Vector3(50 + rand.Next()%5, 50 + rand.Next()%5, 50);

It does nothing. The value _objectMeshFilter.vertices[i] is not changed, it keeps it's old value.

I observed approach of creating some kind of own buffer Vector3 array and then assigning it to mesh vertices array. And yes, that works.

 var mesh : Mesh = GetComponent(MeshFilter).mesh;
 var vertices = new Vector3[baseHeight.Length];
 // now modify vertices array
 mesh.vertices = vertices // copy whole array ?? NOOOOOooooo, sloooow !! :-(  

However that is too slow for me. I only need to change few vertices and triangles. Isn't there any other way to do this than assigning modified Vector3 array to mesh.vertices ? Also I need to do the same thing with triangles. Just edit a little bit.

Every advice appreciated, thanks in advance.