That's completely true, on my previous phone (Nexus 4), as soon as I used Android Stock, without Google Play Services, my battery life increased by around 30%!
I tried to narrow down the cause via battery usage and OS Monitor but since all the tracking and ad related stuff apps use goes through Google Play Services I can't identify the culprit.
I assume routing ad and tracking related stuff and core functionality through there was so Google could lock down their open garden and prevent ad blockers from really blocking them. Bummer that iOS notifications are unusable for me.