Juts a quick animation test with the new clone feature and green screens . Going to work on some 3d model clones tonight aim is create a large crowd are a battle scence .
Pretty trippy! While the new clone feature is pretty sweet (will post some of my own tests shortly...ish...), I've learned that it bogs down quickly with many time-shifted clones. It also caps off at 20 clones per effect, so you've got to double the effect for larger numbers, which increases the drag if they're time-shifted.
For large crowds, I believe you're better off using the particle system (though I've not tried it myself). Here's a tutorial on the process if you haven't already seen it: