User since inception of Brackets here.
In my experience, Brackets is usually much slower than Atom when it comes to open minified files or big files.
To note, Brackets on Windows is orders of magnitude slower than the OS X version of it. (At least, this was the situation when I have switched to OS X one year ago).
Talking about the time they take to “boot”, Brackets it’s much faster in showing you something (a white window), but then it takes some time to load the whole UI, especially if you have plugins.
Atom takes longer to show you the window, but once it’s opened, the UI is loaded in few seconds.