I updated my iPhone 5s from iOS 10.3.3 to iOS 12 through iTunes for a much cleaner install, last night. Right off the bat, the phone unlocked faster, the keyboard and the Control Center both popped up faster too. On iOS 10, I noticed the Control Center would just randomly stutter sometimes even after invoking it for the 2nd or 3rd time.
I haven’t noticed any improvements or disimprovements with the battery life then again it’s only been 12+ hours since I updated it to iOS 12 and I barely use this device and also the battery is due for a replacement considering it’s over 5 years old and it’s done over 1900 charge cycles.
iOS 12 is just as fast as iOS 10, launching native apps on iOS 12 like the Settings, Messages, Camera, etc. will sometimes be faster than on iOS 10 and vice versa. It’s really random, so results may vary but you won’t regret updating to iOS 12. My favorite feature right now on iOS 12 would be grouped notifications that’s the only reason I updated this bad boy.