To make sure it wasn't just my phone being wonky, I asked my work wife to open Pokémon Go and no other apps on his iPhone 6. He was connected to office Wifi at the time, with the app open but not being played for the duration of his test.
In 11 minutes, his phone drained 5% of its battery. In 19 minutes, it drained 8%. Again, if you care about the math: that's 0.45% per minute, or 3 hours 42 minutes of life; and 0.42% per minute, or 3 hours 58 minutes of life. Not nearly as egregious, but consider he simply had the app open and wasn't playing.
Thus far, there's no exact explanation as to why Battery Saver went the way of Vaporeon's Water Gun. But from the sounds of rabid Pokémon players, the latest updates haven't left fans so tickled.