A lot of users complaining about lag on this phone especially with animations. The google gboard keyboard is also messed up with touch issues. I made a couple of changes and now my phone is as fast and buttery smooth as my oneplus 5. Here's what to do.
1. Turn off animations in developer options. Turn them off completely(to unlock developer options tap build number several times in about phone)
2. The Launcher3 launcher is buggy and slow. Use Nova launcher as your default launcher. It is fast and highly customizable. I would buy the nova prime version and its usually on sale.
3. Download swiftkey from google play store and use this as your default keyboard. You can customize this keyboard how you like. I find Large size is best with a dark theme.
Try this out and let me know what you think. I'm on 8.1 with May security so dont know what its like on 8.0