How do people level up so fast?

No I'm not. For new jobs I did the quest til level 30 then went to CDs. Afterwards went to Scarcrows til level 50. Then Jesters. The problem with this is that there are a lot of KSers out there. They're always fighting over a map.

Because you decided to do quests till level 30, everyone who decided to kill at golems/zombies at NLC/whatever else outleveled you and thus they got scarecrows and jester maps earlier than you. Quests are slow compared to training, so you meet more ksers. Also, if there are ksers, I'd rather go somewhere else. There are tons of training maps that are not that bad.

Also, ABs have VERY low damage at 2nd job. They can't even hit 1k very often w/o equips and yet somebody claims to get to 100 in 1 day or less than 10 hours without extra equips? How is that even possible?

2nd job, I 1hitkoed jesters when i was 20 levels lower than them. Only had a 10att wg, a pac, and my characters cards at the time, nothing cubed. I 2hitkoed them with the 2nd job skill...the hit + explosion damage because that skill is just bad...
Getting to lvl 100 in less than 10hours sounds pretty easy with 1.5x.

My mech got to over 116 or so in less than a day. There was no 2x or 1.5x (I think?) or aliens back then. I also didn't do jesters for many levels (I think only 4 levels or so) though I probably should have if I wanted to do it faster. I just trained on the not so popular places. I also had no equips other nub att wgs and pacs, those kind of things, but nothing cubed.
My mercedes got to 200 in 2 days 6-12hours I think without stronghold using 2x and 1.5x (half what kaiser and AB got), had empress set, but nothing cubed I think.
My AB got to 200 in less than 36hours with every exp buff I had, but it's not that funded. Less than 60%dex, the ABs who made it to 200 in a day should've around 300% dex or so.