in eariler versions of iPhone like iPhone 5, iPhone 5S, iPhone 6 if you want to clear notification you have to you have to touch ‘x’ and then you have to tap ‘clear notifications’.
but if you have a iPhone 6S or higher there is a new feature called “3D touch”
this will let you have different kind of responses or actions according to the way you tap the screen, if you press the screen harder than normally you will get a different action or a reply than if you touch it softer.
so you can use this feature in clearing notifications too.
if you want to clear all notifications ( your notifications that are not seen by you will not affect by this, they will be there even you choose this feature ) you can
1.hard press or force press the ‘x’
2.and you will see an option that you can use to Clear All Notifications
This way you can save few extra seconds and also it is very easy to use this.