When I had first installed Atom a few months back, I had performance issues as well, but it does not seem to be the case anymore with all the work they put in the editor's performance last summer.
In my experience atom is really slow at regex find/replace + reindexing projects with lots of files each time you open them. It's going to be nice one day though.
For now my biggest frustration is that Atom does not 'remember' the files/folders I was working on the last time I had it opened, and the Reopen last item does not seem to work at startup.
It's nice, it is a fair bit quicker then Atom (which just seem really sluggish on my system).