Mileage and Gear shift lag in Sonet

So I have got Sonet HTX DCT this Feb.

Gear shift: Initially gear shift was smooth as butter. But later on after a month or so, I observe delay in shifting gears. While changing the gear, it rpm reaches 2k, stays there for 4-5secs, struggles a bit and then changes the gear and rpm then comes down. Mostly observed this in low gears.

Mileage: Right after getting the vehicle, I have filled petrol full tank (~45L) and drove for about 250km. Done and I get the low fuel indicator. At this point I filled the petrol again and it was 37.5L.

So I got 6kmpl (250/37.5) mileage. This is the mileage with two occupants but heavy Banglore traffic. Why is it this bad? I tried switching off AC, tried being less harsh on acceleration. But still same. I won’t rev much and ride in slower speeds.

I’ve got the shell petrol (costliest one) and it doesn’t make much of a difference.

One another observation is D2E(distance to empty) drops quickly. For example, D2E was 100km initially and after driving for just 4-5kms it drops to 50-60 like that.

Am I doing something wrong? Is it a driving skill issue.

FYI. I am a new driver.