The file open times where we are talking about a difference of just over half a second for files which are larger than most web-devs (the target market for Atom and VSCODE) shouldn't make a difference in real life usage.
However, where the big difference comes is in searching across an entire project. Want to find all references to `someVar` it's super snappy in Sublime and 10x slower in VSCode and Atom (of course, it depends on the size of your project as well, but my projects are not that large).
Having said that, I'm still using VSCode, my biggest hope is improved search performance. The Git integration is fantastic.