If you can't stand iOS 13.4's performance on your iPhone 6s or iPhone 6s Plus, note that you can no longer move your phone back to the previous version of iOS 13.
Apple has stopped signing on iOS 13.3.1 which means you can't use a loophole to bring your phone back in an effort to improve its performance.
If your phone is really struggling on you might consider moving to Apple's beta software. It could help your device's performance.
If you're unfamiliar with the iOS downgrade process, take a look at our guide.