I agree Atom will get faster. However, if you look at how it's implemented, you will have to conclude that it will never be as fast as Sublime.
And then all that's left is to decide if you care.
Having written code on a 16-bit PDP11/70, I find that I do care about outrageous waste of CPU cycles. It's cool that Atom exists, is open source, and uses HTML/CSS/JS at its core to do its job. But, it will always be slow, and some things continue to be unbearably slow (trying searching for Space in a large document - it still doesn't seem to recover from that apparent ordeal until you quit and restart). Yes hardware will eventually sort this out, but really, is it worth it?
Maybe will say yes, it's worth it. For now, I say no.
(And I think webkit is one of the modern marvels of computer programming in action ... amazing platform)