I see a lot of people complaining that their phones have gotten slower, and (infuriatingly) I also see a lot of people replying with things like, "my phone is fine, so stop moaning", or "it's due to the latest features of iOS, your phone is too old!", or even "just restore your device and it will get faster and the battery life will improve" (all of which are wrong in varying degrees). My iPhone SE had been slowly getting more and more laggy, stuttery, and just generally annoying to use after two years of daily use. Animations were no longer smooth. Apps were slow to load. It took ages just to be able to take a photo. Everything was just slow and annoying, and I was cursing Apple for not making my device run better. Then I finally hit a wall: I was on a train when my phone went dead with 29% battery power left. I tried to turn it back on, but it said it needed charging. Being out for the day, this was frustrating. Then it finally came back on saying it had 11% battery left. And stayed that way until the end of the day. Something was seriously wrong. Thanks to the latest iOS 11, I used the "Battery Health" feature in Settings to see I was right: I knew I had 84% battery health, but now there was a new message... My phone had suffered a crash because the battery had gotten so old it couldn't supply enough power to the phone when it was doing something intensive. Huh! At this point, the lagginess and stuttering was worse than ever. It was so bad I literally couldn't use a the keyboard without huge delays in typing. So yesterday I took the phone back to Apple, they CS'd me a battery replacement, and now my device is like new again. Super smooth animations, no lagginess, no stuttering, no delays. Everything is fast and responsive again. Holy cow! It turns out that iOS is frequently making huge draws on the battery in normal daily usage, and if your battery is old and can't keep up, then things begin to stutter and lag. It's not the software, it's not the processor, it's not old apps taking up space, it's the battery! OK, you could argue that Apple could try and make iOS more energy efficient by reducing the fancier effects so the phone doesn't get laggy when the battery can't keep up, but this a huge revelation for me. If your iPhone is about 80% health, I'd suggest getting a battery replacement. I feel like I have a new iPhone SE again: Super smooth and enjoyable to use!