Wow. After all these years of taking steps backwards, finally 100 steps forward.
So while it helps improve readability, it doesn't cut down the time to load, the amount of data downloaded, and probably helps improve the battery life only marginally (this is debatable, depending on the amount of active JS, auto-play videos and also at what time offset t the user chooses reader mode after the page loads).
I have in the past noticed that reader mode will not be available of i have killed the JS on a site that enable their print format.