You can now use all the RAM on your system–great if you’re working with big, complex files. Other highlights include:
Gaussian blur received special attention and has been specifically optimized in CS6. As a result, other effects with operations that depend on Gaussian blur have also been enhanced, so you’ll see performance improvements in both drop shadows and inner glows. […]
You’ll notice a nimble, lively touch when you work with multiple artboards and threaded text. Creative tools such as the Bristle Brush have been optimized for both speed and efficiency so you can work fluidly, even when you generate immensely complex designs composed of hundreds of overlapping transparent paths.
And it’s not just Adobe saying it. Here’s Jean-Claude Tremblay writing for CreativePro.com:
It feels as if Illustrator has been re-energized… Modifying these effects in Preview mode is almost in real time. This speed increase and better reliability might not be the sexiest features, but at the end of a day, I’ll be glad I can do more and faster.
The reworked UI also offers efficiency tweaks, including inline editing of layer names (yeah!) and keyboard navigation of font lists.