Oh ya, you guys know what....I just updated to iOS 11.2.1. I then restarted the phone again after update completed.

Now the cellular data as well as wifi speed seems back to normal which is much faster. Remember I complained slower everything after I upgraded to 11.2 back then and it was a rushed release due to 11.1.2 bugs....Apple had optimised everything in 11.2.1 now and it feels so much snappier. Let's see if battery consumption is also improved here.