So... why too soon? I mean, I explicitly started my Eberron game at 3rd level (albeit in Pathfinder) to sorta "skip ahead" a bit. And it's not like the group hasn't earned it: they've conned a bugbear king, toppled a thieves' guild (a small one, but it still counts), clashed with the sinister machinations of a drow mage, and more.
Well, some of it is, I think, that we're in a new edition and I at least am feeling my way around to learn the ins and outs of it, but just about the time I start to feel like I'm getting the hang of things, the group levels up again and I get flummoxed. But I think that's just the surface issue, and there's something deeper underneath, to wit, I've been trained by 3.x to pay super-strict attention to the party levels... and that's something I need to unlearn.
In the faster-looser environment of earlier editions, where each class leveled up at its own rate and a party full of lackeys and hench... uh... persons? was expected, encounters weren't these carefully-structured pieces of art. Encounters were often super-random ("1d10 orcs... so a 100 XP to 2500 XP encounter, and you don't know which? Madness!") and just included whatever the DM thought sounded cool. Of course, that made for parties occasionally getting eaten by something the DM didn't realize would be quite so threatening, but that was part of the game. Hop over to a super-freeform system like Tunnels & Trolls and all bets were off... bathe your sword in a magic fountain you happen upon in a solo module and you might end up with the T&T equivalent of a +4 sword that does a base 2d10 damage or something crazy like that.
But 3.x has tight math in place, designed to help the DM predict how an encounter will go, and prevent the dreaded "accidental TPK". Part of this includes a heavy-handed progression along the level track: in 3.x a party of 5th level characters isn't just a bit more powerful than a party of 4th level characters, it's way more so. An encounter carefully tweaked to be "just right" for the 4th level party, feels like an easy win for the 5th level party; an encounter that was easy for the 4th level party, is a major snoozefest for the 5th. So yeah, you pretty much have to handcraft things to keep them interesting. And every time that level gauge goes "Ding!" you have to refactor everything.
Thus, it makes sense that someone used to thinking this way would cringe when the levels come flying at us like a freight train. But then I need to take a deep breath and remember, "Oh yeah, bounded accuracy." Going from 4th to 5th in 5E is still a significant bump because that's where your proficiency bonus goes up a notch, 3rd level spells and extra attacks appear... but it's not so significant that I have to throw away all of my prep leading up to it. Orcs are still dangerous, they just come in groups of 2 per PC instead of 1 for every 2 PCs like they did at, say, 3rd level. But at 5th I don't have to shy away as much from putting some more interesting things on the encounter list. Tromping around in the woods between Welltide and Pelann, there's been a "1d3 trolls" random encounter that I've been cringing at the possibility might come up, for instance. Now, I'm not so worried about it.
What I can do, however, is to go back and weed out some of the "junk" encounters that would just waste everyone's time, or possibly just narrate through them. "You encounter three ash zombies roaming around the woods. Gimlet, do you want to just channel divinity and blast 'em?"
I do worry, at the current growth rate, that the characters will suddenly be 8th, 10th, 12th level faster than a speeding bullet, and then things might start coming off the rails. Poking around the woods outside Welltide or wandering down into Wave Echo Cave is going to seem very dull by that point, unless those areas have been "civilized" and the characters have moved on to deeper delves and darker dangers. But realistically from a worldbuilding standpoint, that's going to take time. Welltide's defenses are under construction, but that doesn't happen overnight even in a magical setting, and the Diamondheart camp in Wave Echo Cave will need some shoring up before the PCs can leave it alone with confidence.
Basically I need to figure out some way to pad time in between adventures so the world can "catch up" to the PCs' advancement, or slow that advancement down. Maybe story-wise it would make sense to require some "training time" before the characters can actually make the jump to 5th level? Training between tiers does seem like a fairly natural hybrid of the original "train every level" rules vs. the modern "ding mid-adventure" mode.
I'll have to discuss this with the players.