I use DB-33 by Air Music Technology and AZR-3 by Rumpelrausch Taips.
In a rackspace with only one of these plugins, playing simultaneously 10 notes, GP cpu charge is 3% max.
Now, maybe these plugins are not the best, but they are cheap (or free) and lightweight 
@bpeterson : you already read my post in the topic " Optimised laptop recommendations" so you know all about my setting.