How do people level up so fast?

I think the important thing is to have realistic expectations considering your resources. Even doing things mostly optimally (Zombies until 30, CDs until 40 or 50, Jesters until 70 or 80, door to aliens at 70 or 80 if you can get it and MP3 if you can't until 90, and then aliens until 110) on an average class without extra kinds of bonus EXP it'll take me anywhere from 8-12 hours of playing to get to 120 from level 10, mostly depending on how actively I'm playing, how little competition I have on maps, and how good an LHC party I find when I reach 110. Even so, that's with a lot of luxuries that a long-time player would have that a newer player might not have (full set of link buffs, evolving rings, level 30 empress blessing, 6 S rank cards, attack work glove, etc).

If you don't have conveniences that make killing higher HP monsters like Jesters and Aliens a quick thing and you're not playing at times where you don't have to compete with other players, you're probably putting in similar levels of effort with much less reward.