Not only the start up time, it seems to hog memory and bog down my Macbook Air (4GB) after using it for awhile. I'm not entirely sure if it was the length of time I had Atom open or the size of the project. I usually have Chrome running with a small Vagrant box. After a few hours it was unbearably slow to even switch between open files.

I closed Atom and my memory pressure dropped considerably. Installed Brackets and the pressure is still low even with Chrome and a box running yet.

This probably isn't an issue for people with 8+ GB of ram though, unless it actually has a memory leak.