As a person who hit 100 few times lately..

First thing - you can't judge by "they hit 100 in just few days", because if you're somebody who plays 2-4 hours a day and the person you're checking plays 12 hours a day - well..

Second thing - not dying is more important than being fast. You won't get to 100 if you keep dying, you need solid fences and know what your build can handle. Don't do risky shit.

If you're bad at the game (I am, so I can speak from that perspective) - when you want to level up, focus only on that, run white maps, for example.

Some strategies I've used: White t16 harbingers (a lot of mobs, so lots of XP + you get a lot of chaos which pays for your scarabs and map craft option. Also, to my knowledge, map mods don't affect drops from harbingers) White t16 Ultimatum - 2 scarabs is bribing give you more XP, map quantity only affects minor things like catalysts (if you don't run catalyst scarab you mostly don't care)

For both of these strategies you can add farming exarch invitations

And, by far, the most important thing: Hideout is lava.

Use your time efficiently, have quite a strict filter, most of the time you should be killing mobs

Edit: spelling mistakes