You can get the version of Chromium that Atom maps to by opening the Developer Tools and going to the Console tab. Once there you can enter
process.versions to get a list of the versions of various components underneath Atom. Here’s the output from my recent dev version:
So, as of last night, Atom was running on the engine from Chromium v47.x.
As to “why the lag,” we only use stable versions of Chromium in Atom and Electron. Therefore, it has to be released first. Once it is released, we integrate the Chromium engine into Electron. Periodically, we update Atom to newer versions of Electron. Electron is changing quickly and changes to Node and Chromium, while they may not impact a majority of Electron users, may break functionality in Atom … which then need to be worked around. In the interest of stability, we don’t move to a new version of Electron unless we’re reasonably to very certain that it won’t break Atom for our users and package authors.
To my knowledge, we’ve never made any guarantees that Atom or Electron would be using the latest or even relatively recent builds of Chromium or Node.