The biggest reason why any Adobe product slows down, other than Ram, is because they are scratch hogs.
The fastest way to work with them is to keep 1/3 to 1/2 of your hard drive empty for scratch space so the software isn’t seeking for blocks to dump history information. That also means running the maintenance scripts once in a while (both Mac and PC.)
Secondary hard drives are an option, but they are only as fast as the connection you are using. The startup drive is the fastest option. No movies, no music, no games, no nuthin on the machine that does your paying work.
(Though, I am concerned with the new SSD hard drives and the write/rewrite life…)
It’s counter-intuitive, but sometimes shutting off GPU performance helps too.
And Ram of course. 16 gigs is about the minimum for current CC products if running them all together. That said though, you would have more trouble in Illustrator with gaussian blurs (glows) than you would with a gradient mesh.
Are you combining effects in some way?