In the Energy Monitor on my Macbook Pro 13-inch 2017 base model, it shows high Average Energy Impact (~75) for FaceTime. Since the number is the same (a ~0.5 difference) to to another app I was running coincidentally (League of Legends.app), they must be correlated.
Alas, I only noticed this more than (!) 15 minutes after I had shut down League of Legends, unplugged Ethernet and display, unplugged the Laptop from charger, and took the Laptop into bed to write E-mail. The fans were still running and it didn't seem to cool down from my gaming session.
As soon as I quit FaceTime.app, it cooled down and the fans turned off.
If I open FaceTime.app now, or in the past, it doesn't use any significant resources at all.
Why did the energy impact seem to hang on Facetime.app? Shouldn't it just be the processing-intensive League of Legends game that is using so much energy? Once the game was off, why did Facetime.app continue to burden my system?