Tiagra lever extremely stiff on new bike I bought myself my first real bike (Conway GRV 600) three weeks ago and was pretty happy with it. My GF decided she wants one too and got herself a Canyon Endurance Bike (cheaper than mine but same Tiagra group). Upon trying out her shifting I noticed that hers is extremely easy to use whereas on mine I have to apply quite a lot of force and by quite I mean like a ton more. With hers it's possible to go like click click click with an easy hand move, whereas on mine I sometimes struggle to get even one gear up at a time. When I originally got the bike I didn't know that it was not supposed to be like that, however now that I have something to compare, the differences are striking. 

Some facts :


The bike was new but sold to me with a 20% discount on the retail price 
I have ridden for around 400km since I got it, almost exclusively on roads and always in the dry - so I think my cables should be fine 
shifting up the cassette with the rear derrailer (that is into easier gears) is what requires a lot of force, going down the cassette is really easy and fast on both bikes 


Any suggestions what this could be caused by? Is it due to different cable routing, or might it be that the bike has been sitting too long somewhere and the tension on the cables is bad?
As mentioned in comments you should take the bike back to the store and get them to fix it. I wouldn't even attempt a repair because disassembling anything introduces the possibility the store will decide you caused the problem.

However, initial steps for diagnosing the problem if you want to:

I think it's highly unlikely the that cable routing is the problem because the cable would hang up on both up and downshifts.

You can easily determine if the shifter or derailleur is the problem. Detach the cable from the derailleur, work the shifter. If downshifting is now free the dearilleur was the problem. If still stiff it's the shifter.

You can also attempt to manually move the derailleur from the smallest sprocket to the largest while holding the rear wheel off the ground and pedaling (you need a work-stand or a friend to do this, or possibly put the bike upside down on something that will not scratch the saddle or hoods). Be careful not to catch your fingers in the chain.