Subaru Impreza Fuel overconsumption

[Wongz]

I have just bought a second hand 2010 Subaru Impreza with 1.5L engine from someone and have noticed fuel consumption is bad. Like on average, it's covering above 10km/L ..with lower 8.9km/L or higher 12.3km/L. This is driving withing 100km/hr. And again, it hesitates or drags like it is suffocating in some instances while I drive. I have changed air filter, oil filter and engine oil plus suspension. No improvement .

Is this normal for this car? Help a brother.