@Abhishek_TV, apps scroll smoothly, even media rich websites. Other launchers work well. So the problem has to be somewhere in the default Launcher.
If it's of any help to users, I found that Microsoft Launcher is both smooth and offers double tap on screen to lock (with 2 options, instant lock where fingerprint cannot be used to unlock, and a delayed lock where fingerprint sensor can be used to lock).
This can be an interim option till the stock Launcher problem is sorted out.