Very stiff front derailleur
First of all, if the bike is new and there is a problem with it, just take it back to the store where it was bought and ask them to fix it. Most bike stores in the US offer some sort of tune up after a few weeks to deal with any problems that crop up, re-adjust as cables stretch etc.

If you need to diagnose and fix it yourself:

You have ruled out the shifter, so the resistance must be in the cable or the derailleur itself. I doubt it's the derailleur spring being too stiff. The derailleur lever mechanism and springs are designed to have approximately constant  return force across the range of movement. The inexpensive Tourney line is has some stamped sheet metal parts, there may be a burr or defect that is preventing the derailleur from moving freely.

To check the cable, disconnect from the derailleur at the pinch bolt. While holding it taught at the end work the shifter and make sure the cable moves freely and is not hanging up at all.

With the cable still disconnected, you can manually push the derailleur cage from the small ring to large ring position to see if there is any resistance. It helps of you can pop the chain out of the cage, there should be a small screw that you can undo to open the cage and slide the chain out.