I use it daily as my IDE. I find that startup time is indeed slow as a dog, but once it is up and running (and it's typically open all of the time, so startup time isn't a real issue) it runs fast enough. At certain times I notice small delays on more complex ops compared to sublime, but nothing significant. The main pro's for Atom, for me, are the way the eco-system is developing, the open-source nature of it, and I'm slightly more happy with the interface. If push came to shove though, I could be happy in either sublime or atom.
The difference in speed in these editors is so small that, if you find it's having a measurable effect on your productivity when coding, you may want to re-look at how you measure productivity or indeed how much time you're churning out code versus other development activities (thinking, planning, testing, talking).
The issue with speed in an editor isn't so much that if your editor is slow that it makes you code faster, but that if your editor pauses, it interrupts your thinking and prevents it becoming very natural and quick to use, like an extension of your mind.
So it's not so much '60 seconds of waiting for the editor in a day', as '2s delay opening that file so my brain switched to thinking about something else, slowing me down for 10+ mins'
The difference in speed in these editors is so small that, if you find it's having a measurable effect on your productivity when coding, you may want to re-look at how you measure productivity or indeed how much time you're churning out code versus other development activities (thinking, planning, testing, talking).