Electron is not anything much different than Chromium Embedded plus some native API hooks, so it's not the Electron parts that eat the resources, it's Chromium (and basically, it's the fact that the app is basically a web app that needs a whole layered and bloated rendering engine -DOM- and a slower runtime - js - compared to native).
Doesn't electron include the nodejs runtime as well?
Yes, which is just v8 + some system interfacing libs, not something that is slow/memory hungry by itself anymore than v8 alone is.
It would be slightly less since it is just Chromium versus Chromium + Node.js (as in Electron).
No idea. I personally have little tolerance for the amount of resources that Chrome uses, so I stay far away from its derivatives, which I have found to not be meaningful improvements.