Check the cable housing. If the routing is too convoluted, friction can delay shifting. It's quite common for bikes to be assembled without trimming the housing to a length allows for the least length of housing needed route the cable properly. Too much housing increases friction both by the excess length creating more friction and having to go through complicated curves and loops on the way to the derailleur. Less housing is better, but only to the extent that you can stll turn the bars without binding.