Three.js - Buffer geometry particles, need to animate random groups of particles in system

Question

I have a PointCloud with a large number ( 200K+ ) of particles. This PointCloud is constructed using THREE.BufferGeometry, with each vertices position set using Float32Array( numParticles * 3 ).

I would like to animate the x,y positions of randomly selected clusters of particles every n number of seconds. This process needs to be as efficient as possible — so looping though all the particles in the update() and selecting random clusters to animate, is too computationally expensive.

Is there a better way to achieve something like this in Three.js ?