If you can't stand iOS 13.3.1's performance on your iPhone 6s or iPhone 6s Plus, note that you can't move it back to an older version of iOS 13.
Apple has stopped signing on iOS 13.3 which means you can no longer use a loophole to bring your phone back in an effort to improve its performance.
If you really want to get your device off of iOS 13.3.1 you can move your iPhone 6s or iPhone 6s Plus to beta software.
If you're unfamiliar with the iOS downgrade process, take a look at our guide. It'll help you prepare for next time.