Derailer systems only absord energy in the chain and bearings, and a well maintained system can maybe lose 1.5% of energy. Internal hub gearing can lose around 15% energy.
The shop keeper won't tell you this, he just wants to sell you a bike. If he starts talking difficult things, buyers might just go to the next shop to buy what they had already chosen.
Funny, I thought it was my three young kids that did that.
I always had a mountain bike growing up. Having now transitioned to more of a road bike, the difference in rideability is pretty stark.
I guess it could be the derailleur, but I assumed it was the tires and gearset that made it a pain to get places. I also have noticed a night and day difference when my tires are properly inflated. I just replaced the tubes and tires so maybe I'll have better luck now, but previously I needed to refill the tires basically daily to keep them in the right range.
1) using latex inner tubes. They’re lighter, but don’t hold air so well. Fine for racing, impractical for normal use
2) a slow puncture
4) badly seated valve tip (dirt?)
If you actually take a look at your derailleur system, you'll notice that a) the chain is not straight in all but a few settings, b) it has a somewhat complicated arm with an extra sprocket to keep the chain tight. And it is harder to protect it with a guard, so you end up with more dirt in the system if you use it every day.
But yes, internal hubs are heavier, generally somewhat less efficient, and better protected against the elements.