Apple doesn’t automatically clear useless data from the system, so it is often helpful to do a factory reset from time to time.

If the battery hasn’t been changed (<80% as seen in settings) the system will throttle the speed to preserve longevity (supposedly). Battery replacement can be costly, however.

The final possibility is that the iOS you updated to is simply too difficult for your device to run. Unfortunately in this case nothing you can do will help without downgrading.