Sorry, but it's heavy on the cpu in either case, albeit "notably lower" without the Christmas Lights.
I've tested it (without the lights) with Safari and Firefox. The former sees approx 70% cpu use and the latter around 100%, both cases with no other applications running. Move off the site or switch off the snow and the cpu usage drops down to a negligible level.
I'm not fluent in javascript, or in particular the nuances of its thread handling (my language is the close relative java) but having had a quick look I think the code might have a bit of busy waiting in it, meaning that the code loops when waiting to do other things, which makes it hungry for cpu cycles. I once had similar issues with an applet I embedded in a webpage, even when paused.
If you open Task Manager in Windows (or Activity Monitor in os-x) you can see the difference with and without the snow.
Not half wrong, just watched Firefox jump up and down in task manager. It bounces back between 4% and 96%. I can also see java going up and down like a brides nightie, in between use mind you.
Had a really good look at the javascript and, apart from spotting a couple of slight gains to be made by trimming some unneeded 'if' statements and perhaps using stacks to track active and inactive flakes I couldn't see a lot to do to.
Then I had a look again at the author's web page and he's slightly updated the code to use bulletpoints for snowflakes instead of image files - and this significantly reduces the cpu load - more than halves it using Safari on my laptop. Saves hosting the snow image file too. Version 1.4.20091115.
Oh, and you can muck around with this.snowColor too if you want... (try going from fff to d00)
Yes, although there is a compounding effect when it runs on a forum page.
When I run the script at http://www.bloodforthebaron.com/snow/sc ... owstorm.js in a simple blank page with black background I get CPU usage around the 30% mark. A forum page comes closer to 50% average - it's come down from the 70% seen before, but it could be that the browser's need to re-draw pixels on the page where the snow has been is having an effect (which I think is out of the author's control).
Altering the maximum number of flakes and active flakes might help (but there's a payoff in appearance) but beyond that I'm unfortunately stuck - sorry.
