If you don't like how iOS 12.4.1's performing on your iPhone SE you can try downgrading to iOS 12.4.
Apple is signing on the previous version of iOS 12 which means you can use a loophole to drop your device back to iOS 12.4 in an effort to improve performance.
Unfortunately, there's no longer a way to bring your device back to iOS 12.3.1, iOS 12.3, iOS 12.2, iOS 12.1.4, iOS 12.1.3, iOS 12.1.2, iOS 12.1.1, iOS 12.1, iOS 12.0.1, iOS 12.0 or iOS 11. Apple has stopped signing on those updates.
If you're unfamiliar with the downgrade process, take a look at our guide.