Gruber linked to this, so you've probably seen it before, but this article by Craig Mod is great.

But why is slow bad? Fast software is not always good software, but slow software is rarely able to rise to greatness. Fast software gives the user a chance to “meld” with its toolset. That is, not break flow. When the nerds upon Nerd Hill fight to the death over Vi and Emacs, it’s partly because they have such a strong affinity for the flow of the application and its meldiness.

I completely agree with most of his points here. Software that feels fast simply captures me so much more than anything with constant lag. Drafts, Overcast, Reeder, Tweetbot, Ferrite, Sketch, Apple Maps, and Safari are all apps I use specifically use because they feel fast and let me get things done as fast as I can think about them.