Why are my race times slow?

It's a 12 year old car. Unless this car has extremely low miles on it (ie. under 50K miles), it's not putting out the same amount of power/torque as it did when it was new. that is the likely culprit. 

Strip it down. It's too heavy.

Quoted 0-60 times you see published by automotive publications are often corrected for altitude, temperature, humidity, barometric pressure etc., in the name of "standardizing" all of their published test numbers.  Conveniently, that almost always also makes their cars look faster than they actually were before corrections were made...

Another reason you probably can't achieve their time - most car magazines use a "run out" distance before they start timing.  In other words, the timer may not start until the car has moved ~5 feet or so.  If you're timing from the instant you stomp the gas pedal or have something that is starting the timer for you the instant the car begins to move, that will add several tenths of a second to the 0-60 time as well compared to how automotive magazines test for "0-60" times. 

Other factors - the car is 12 years old and they wear out over the years which can make them slower over time.  Also, if you've got anything in the car besides a full tank of gas you'll be accelerating more weight than the test car did when it was timed at 5.5-6.2sec 0-60mph time.

To get close to the quoted 0-60 time, you need the car as light as possible, that would include an almost empty petrol tank and ( don’t take this personally), possibly someone lighter than you .

Also someone with more driving talent. 

Ignore the suggestion re auto transmission, as an amateur, you will almost certainly get better times with an auto.  

You do not have a race car.  Stop worrying.  And those times are when the car was brand new.  If you want to improve your times, put a V8 and throw away that automatic transmission.

Could be a number of things,: altitude, temperature, driver, timer, 12 years of use. brand of tire and age and pressure, what is in the trunk.

if you actually have that car, then it's probably because you have the automatic transmission. manual transmission cars are generally quicker because they are lighter, have less parasitic losses from the torque converter, and different gear ratios.(modern dual clutch transmission and automatics with launch control can be quicker than manuals, but yeah on slightly older cars, manual is generally quicker)

online, it looks like the manual G35 is the faster version. either way it doesn't matter that much. it's a commuter car not a supercar. but i say that as a person who owns a 1.6 liter 4 cylinder and doesn't care about speed

you  gave  the  wife  a  good  serve  last  night?

could be anything from:

slower reaction speed,

quality of track,

strange fuel mixture,

mechanical problems with the car (worn things etc.),

even as simple as tires

You would do well to have car inspected to get it up to specifications if need be, than train yourself to faster reactions, better control of rev and shifting (possibly most common?).

It may not get you on par with the fastest time but it should help.