Question....why does everyone want to speed level so quickly? I'm about to jump into Diablo 3 in my first season, and it seems like everyone has a specific goal of blowing through. Is it to work on getting high level gear as soon as possible or what?
To add to this, you have to understand that it's the 6th season, a lot of people have played every season, or most seasons, and have leveled every class already, or most classes. Leveling doesn't change in D3, it's the same everytime, due to the set rate of skill acquisition. If I level a DH again, it's basically the exact same process as the last 5+ times I leveled a DH. I mean it's slightly different than vanilla in terms of gameplay, but not even that much(and that was several years ago with a lot of skills changed). Mostly I'll just chakram/multishot stuff until max level, with a basic vault/prep/companion/hatred builder with hatred rune and one skill that depends on my current gear and stuff.
That means that if I were to play DH again, the leveling process would be extremely boring, it's not new, or interesting. Difficulty is low(and increasing difficulty just makes everything takes forever to kill without really increasing difficulty), and most of the game is about getting sets and climbing GR levels, so the faster I get to max level the better.
You can actually consider Paragon and Greater Rifts levels as the actual progression in the game. You reach 70 asap to start the real season progression.
Note that if you haven't played the game to cap, or you haven't played a particular class to cap, it makes perfect sense to instead take your time. There isn't a need to rush. If you haven't played since the release of RoS or even vanilla, there's a lot of new stuff to do and find.
Even if you don't rush though you'll reach max level in a day/a couple of days, it's just that kind of game now. Unless you do like story or whatever then it's kinda slower, but just discovering your class and doing bounties instead of spamming rifts and not doing the efficient start or getting leoric's crown or leveling in a grp or whatever, it still only takes like less than 10hours to reach cap. Around 5 if you don't do story and don't play on a difficulty that's inefficient, even without optimization and counting various breaks.
