For me, the issue with Atom and speed has nothing to do with large files. It's that even with small files, it's often just slightly unresponsive--not that it freezes or anything like that, just that lots of things take a fraction of a second longer than they should, which becomes very annoying in the long run. In my opinion, this really shouldn't have to be a problem today... on the other hand, it seems like a lot of people just aren't bothered by this kind of thing.