OK, I can here by confirm that using MM 2.1.0 + Electron 1.4.15 improved performance massively. Each new transition now only uses 10-13% of CPU power (and very, very smooth). Even better than earlier report of 40%. For the specs of my pi 3 b
hardware = BCM2835
Revision = a02082
micro sd card - 16gb - class 4 (that’s right, only 4)
mouse + keyboard currently plugged in - no lag
OS = RASPBIAN STRETCH WITH DESKTOP
Thank you so much @Mykle1 for sharing your finding. Otherwise, I would be still trying to narrow down a potential work around.

For those who not sure how to do this:

1. Create a fresh install of RASPBIAN STRETCH WITH DESKTOP, connect to wifi, then run sudo apt-get update.
2. sudo apt-get install npm
3. sudo npm install electron@1.4.15 -g
4. WAIT PATIENTLY FOR ELECTRON TO LOAD IN THE BACKGROUND
5. a new line appear indicate electron has finished installing
6. Go to your download folder, exact Magic Mirror 2.1.0
7. Go to the newly exacted forlder, open another folder named config
8. Change the file name Config.js.example to Config.js inside the config folder
9. open terminal , cd Download/, cd MagicMirror-2.1.0, (OR whatever your location for the Magic Mirror 2.1.0 package is), npm install && npm start
   9.5 If encounter error “404”, try “npm install npm@latest -g” (NOTE: some users experienced issues since they do not have the latest npm as global variable) @Reotch2 @EventHorizon
10. DONE!

Hopefully I will be able to return my pi 3 for a better hardware version to use with the latest Magic Mirror package but here is an acceptable work around for the time being.

@Reotch2 wow… it seems different people have different version of npm installed and you need the latest one. I will add that into the steps. Thank you for verifying. Good luck on your project!
As for electron version you can confirm by trying “npm list -g” or “npm list” depend on global variable settings of your packages.

Yeah, many didn’t update npm and have no problem whatsoever, only two users reported having 404 problem and only 1 user tried to npm install npm@latest -g and it worked for him. I made a note in my guide up in the thread in regard of this issue.