Updating mesh.vertices or mesh.colors slow on iOs.

I appear to have hit a performance bottleneck. My target platform is iOS. Testing on iPad first Gen.

I would like to distort a 128x128 grid (16k vertices) on every frame, Everything is running at 60fps until I set the mesh.vertices. Performance drops to 25fps.

I have also tried setting mesh.colors and distorting the vertices in a vertex shader, but this also creates the same fps drop. (Vertex shader runs at 60fps with baked vertex colors).

I've test a 64x64 grid, fps 33. and 32x32 grid which achieves 60fps again.

Is their any way that I can, ether update a meshes vertices in an efficient way or get the vertex color array to the shader in a different way?