I don't know if you've already had a chance to finish this before now, but here goes....
To change the speed of the falling particles you'll want to edit the @vx
and @vy
on lines 57 and 58 of that forked Pen of yours, something similar to:
@vx = range(0,1)+8*xpos-5
@vy = 0.2*@r
I found that altering the DOP (and Y value to compensate) gave something a little more 'believable' too:
@dop = 0.001*range(1,5)
@y = range(-100,h-@r2)
With regards to drawing more 'realistic' snowflakes, I personally prefer seeing shapes closer to "circles", since the actual flakes themselves are harder to see in real life. But if you're keen to work on your original idea I can point you to HTML5 canvas fractals.
A good example of what to look into if you're looking for something simple are Pentaflake fractals. If you really want to blow people's minds then I would suggest looking into the source code of this HTML canvas fractal application, here.
I hope this helps :)