It’s not really about time-management, just how many hours you prioritize sitting in front of your screen instead of spending it on other hobbies or family time.
10-15 hours from level 1-100 playing solo? I have to hand it to you then, that’s an impressive speed. I have a hard time believing that’s even possible, but I’m open to seeing some proof or examples.
And I’m not agreeing the game should become even “easier” or whatever, I have a really fun time playing that barbarian. I hope they don’t nerf anything, but rather work on buffing the other classes to match and maybe increase end-game boss-health so they don’t go down so fast at high levels (yes, I heard about 2-hit Duriel-kills etc. which is a tad overboard 
.