The application itself loads pretty quickly for me, but using the adjustment brush tool is frustratingly slow. There's a good 5 second delay between when I do something and the image updates to reflect my change, and if I move the mouse within that 5 seconds, the changes are also applied to wherever my mouse went, even though I've already taken my finger off the button.
Oh well. I was looking for a reason to build a new machine anyway. =)