Atom is noticeably slow for me on a modern rMBP, too.

I'm using version 1.1.0.

It used to not even let me open anything but the smallest files. I think the limit was a mere 2 MB!

I do not get that denial any longer but it still feels slow especially for larger files.

Sometimes I have to open 500 MB text files. Atom cannot handle them, while they open nearly instantly using MacVim.

So in these modern days I use MacVim more than I use Atom.