After picking up my iPhone 6 from Verizon I went home and added all my info and Install Swiftkey. This app was something I was looking forward to.
But after I installed it. I could just sit and watch my battery drop. I think my battery lasted 4 hours.
So after deleting the app and setting my keyboard back to English. I thought I would try another keyboard app. So I installed Swype from the App Store. I saw the same result as Swiftkey.
So if you have bad battery life on iOS 8 and a keyboard app installed. You might want to remove it, and see what your battery life is after.