With 4 adults and AC I felt the same way, It does struggle a lot when I compare it with my previously owned JDM Alto 2015. The main issue here is engine settings, as local is set to give more mileage than power I dont know if Pak Suzuki can manage these modifications or recommends them.
But the irony is that the Japanese Alto used to give same mileage of 21-22km/l with those settings... and Hamari local ki power kam ki taake ziada mileage de, lekin phir bhi Im getting 20-21km/l lol...