Are you on 10.3.1 or 10.3.0, with 6T, if you are not on 10.3.1 you can update via Oxygen Updater, this may fix the issues.
Also try the suggestions below for battery issues.
Just in case there is an App hogging resources, causing lag, Battery drain or freezing, clear your recent apps, also try setting 3rd party apps to Optimize in Settings Battery, only leave the ones you need run in the background or to notify you such as messages, email, Google Services, oneplus etc on Intelligent control.
Android may take a few days to optimize those apps after setting them to Optimize depending on your usage of those apps. A10 has its own learning curve.
Also charge your phone to full and restart it after optimizing, and see if that helps.
Check App battery stats at about 50% and see if there are particular apps using more battery than necessary.
Battery Calibration/Heating Tips: How To Improve Battery Life !!
https://forums.oneplus.com/threads/...ting-tips-how-to-improve-battery-life.556988/
A definitive guide to everything that affects smartphone battery life
https://www.androidauthority.com/smartphone-battery-life-drain-causes-1071423/Click to expand...