I noticed the slowdown. Never believed in planned obsolescence so I though it could be an optimisation problem in iOs 11 but problem persisted after several os updates.

I finally decided to upgrade to an X because how bad it was. Turns out it is throttling by as much as 50% (I still have the phone) so I could have just spent 79$ instead of buying a new iPhone.

The same will happen to a lot of people.