my dear have been i lots of vitz....thir is nothing wrong with yr car.....
its vvti doesnt means u have a speedi engine...this technolodge basically burns fuel 3 times as normal engine burn 2 times
u are running car on cng plus AC and its transmission auto...so it will be slow
and how u say slow...have u compare it with anything then do let me know
the plugs u installed are use less without wires
i m currently having same plugs with hot wires which actually had lot of power in 2nd gear