Working myself into the deeper layers of 5th Edition, I noticed that defeating enemies in battle gives the PC massive amounts of experience points. If a party of four PCs were to only fight opponents with a CR equal to their current party level, it would take only 6 such encounters to get to 2nd and 3rd level, and after that 11 encounters for any further level. Though I have been told that in most cases, a single opponent of equal CR is a pretty easy fight.
Now at this point I want to repeat that I don’t believe in setting up encounters tailored to the current party size and level. While megadungeons and hexcrawls are leaving me completely cold, I feel like non-linear, open-world environments make the most interesting stories and experiences. When there are no places the party has to get to, and paths they have to take to get there, then there are no grounds to assume that the GM will take steps to ensure that the party will overcome any given obstacle. As players, you have no assurance that an obstacle can be overcome reliably by the party at their current strength, or that there any paths to their destination that they can take safely at their current strength. This puts it into the hands of the players to judge what dangers they are willing to risk, and when to try trickery or diplomacy over direct confrontation. Of course, to make this work, it is necessary to make it possible for players to judge the dangers ahead of them, and to attempt a retreat from a fight that turns bad on them. Otherwise the GM is just forcing random fights at the players, without giving them the agency to choose the level of risk they are willing to take. In practice, this primarily means not setting up ambushes by hard hitting opponents, and not cutting off the party’s only escape route.
Another thing is that I don’t believe the math to calculate CR being in any way reliable. But for the topic at hand, let’s continue with the assumption that challenge ratings and the encounter building guidelines were actually precise.
Under hypothetical ideal circumstances, a party of four characters fighting only single opponents with a CR matching their own level would, according to the tables, get a new level in six and a half days of adventure after roughly 10 fights. On first and second level even faster than that. This is of course ridiculous, and the designers did acknowledge that. Fighting only single opponents makes it very easy on the players. If multiple opponents attack all at once, the party will be able to fight significantly less of them on a day before they require rest to regain their strength. So if you want to fit more fights into a campaign that goes up to only 10th level for example, you could make opponents usually appear in groups of 5 to 10. Going with the hypothetical tables, that gets you only half the amount of XP before you statistically run out of power. Or 13 days of adventure with 20 fights. And when you think about it, for lots of common opponents it makes perfect sense to fight in groups. From the perspective of bandits or monsters, they want to win fights and survive, not put up a good fight before being defeated. And in a world where there is always a bigger beast, those at the lower end have every reason to band together for their safety. Just like PCs do.
But that’s still for the assumption of fights taking place in open fields. If you want to get the most out of your monsters, make them make their stands in places that are to their advantage. And frankly, fighting with the environment instead of just ignoring it makes every encounter at least twice as interesting and fun. Why would you ever not want to do that? Eyeballing the tables, I would say that if you make your opponents fight as teams and using the terrain to their advantage, it could probably take the party up to 30 “level appropriate encounters” to make it to the next level. This really isn’t insanely fast anymore. It actually seems pretty slow to me.
Though on the other hand, you sometimes really want to have big bad boss monsters. But I don’t think these really need to be that common. Most of the time it makes lots of sense for them to surround themselves with plenty of minions. After all, they also want to stay alive and should know that they are at a big disadvantage when they are outnumbered.
All in all, I think my initial suspicion that characters are gaining new levels way too fast in 5th edition doesn’t really stand up to looking at the tables. Now, of course I don’t trust the tables and I don’t plan on actually using them beyond the first couple of starting locations to get a first feel for the system. But still, I don’t think I will have to worry about making adjustments for how many XP characters will get for defeated opponents.