Atom may always be slower, but the speed difference could eventually become negligible. If ST3, in it's current form, existed 20 years ago, people would have been making the same argument about Vim vs. ST3.
Actually, rendering is often faster in Sublime Text than in vim. I am a vim user and use the 'cursorline' and 'relativenumber' options and until recently I could notice rendering slowness on OS X and Linux. Things seem to have gotten better recently (either in vim or due to hardware advancing), but I never saw such slowness in Sublime text.
(I wanted to like Sublime and have a license, but Vintage mode is a too leaky abstraction/emulation for me ;).)
I hope so, but it'll require significant improvement. From the command line, "subl ." loads almost immediately, whereas "atom" can take 10+ seconds. (on a MBP with 16gb + SSD)