> I rather have shorter battery life with same performance so it's more noticeable that I need to replace the battery.
I agree, but something is odd about this whole situation.
I have an iPhone 6, iPhone 5, iPhone 4, a 2nd generation iPod Touch, and several Android phones.
The iPhone 6 is the first one I've seen randomly turn itself off when the battery level was below 30%, all of the others would run normally even when the battery condition was poor. I could continue using them normally while the battery indicator ticked down to 1%.
While my iPhone 6 no longer randomly turns itself off, it frequently doesn't respond for 15+ seconds when typing a letter on the keyboard or tapping a text field, it's essentially just as unusable as if it had turned itself off.
What makes me suspicious is that I have a 2nd iPhone 6 here at the moment with a similarly aged battery in fairly poor condition. It doesn't randomly turn off, and the performance difference is immediately noticeable, it's actually usable. The only real difference I can identify is that it is still running an older version of iOS.