This was before the XP nerf. I play default settings, but set air drop to once every 7 days, turn on the air drop marker, and set max alive to 16 or 24. Use molotov for crowd control on horde night with 24 zombies and get 5-10k xp right there with one shot.
Mining was super OP, at level 50 with all the perks and a decent steel pick you could level every 30 seconds. (I'm sure its this way in 17 yet). I play default length days, 60 minutes I think they are. Bear in mind every zed I see I kill traveling and I'd have nerd goggles for XP bonus. It wasn't uncommon with the zed density in the POIs to level up on the way to the POI, be at the POI when a wandering horde showed up outside, and level up 3-4 levels in the POI, come home gain more xp, turn in the quest and level again. So if you quest constantly, kill everything, mine most the night and do some daily upgrade to your fort, you could level super fast. Like level 20 by day 7 for sure, and then it got faster and faster as you could kill zeds faster, you are getting more xp.
You can see my current level in the screens I posted on twitter. Its 1 more than your max HP now. Its definitely slowed down to a good pace now IMO.
We've completely rebalanced XP since 17.4 so that each activity is fairly close.