This is something I think is very subjective. I’ve time and time again played games where I thought the leveling was incredibly slow. Then the next character was faster, and the third even faster than that. Not only because of experience gained, or better items handed down. But because of the perception of the time it takes to grind the levels has changed.
The grind to 100 in D4 is quite minimal compared to any level based grind game. The only person who required two weeks to become level 100 are 2 full-time job working single parents. Streamers and hardcore grinders did it in what? 10 hours?
I don’t track my time, but it wasn’t 10 hours. I did it in maybe a week . I rerolled from 91 Necro, to 84 Druid and then to a 100 Barb. I did not grind it either. The key is getting a good build going around 55-60 since you have access to tempering and ancestral. Any trash with tempering will make you annihilate nightmare dungeons. You only need enemies to be 10 above you to get full XP.
You can easily do a dungeon in 5 minutes (a lot less if you are fast). One dungeon, even on high level, is a BIG ol’ chunk of your XP bar. Even at level 99, you only need like 3 NMDs (don’t quote me on it) to get to 100.
Just to prove a point, I loaded up my 84 Druid I haven’t played in at least a week. Took a level 51 NMD where the monsters are 21 levels above me (only need to be 10 for max XP), and I rolled it with my very VERY slow Druid who has no movement speed increase in ~5:30 while picking up ALL the loot and opening ALL the chests. I got ~45.5% of a full level.
So although you may perceive it as slow and grindy. It’s anything but. Throw in Holy Bolts elixir or Momentum to speed things up, and you’re level 100 before you could have finished your post. 
And since no one asked, I will answer the post 50 levels as well. The paragon levels hold more power than your 50 first levels. Adding the passives and glyphs to your build is as important as anything else. So to say that they are “pretty much just cosmetics” is incorrect.