Part of an ongoing series of 5e Rules notes.

So this is less rule than game design philosophy. It feels a little Inside Baseball, but understanding it is fundamental to understanding a lot of the reasoning behind the rules in 5e, and in why the game behaves the way it does.

What is the “Action Economy”?

In short, action economy means what a character (or NPC or creature) can do each turn. How many attacks can they make? How many abilities can they use? How many spells can they cast? A lot of the rules I’ve researched in here orbit around that concept of action economy.

Essentially,

the larger your action economy ⇒
the more things you can do in a turn ⇒
the more powerful you are

And that’s true for individuals, as well as for groups.

• A big part of character advances are adding more attacks, more Bonus Action options, etc. Similarly, more powerful monsters have more attacks and actions in a turn (including legendary and lair powers).
• All things being equal, the side that has the greater numbers of combatants has an advantage in combat, because their action economy, the opportunities they have for success in combat, is greater.
• Bounded Accuracy , as one person put it, “makes everybody dangerous no matter how weak but does so at the cost of making everyone vulnerable no matter how strong.” Which means, by implication, over time a bunch of weak (but dangerous) characters can overwhelm a strong (but vulnerable) one.
• PCs often have advantage in combat because encounters often have more PCs vs fewer (but individually more powerful) enemies. Sure, that monster can do three physical attacks, or maybe a big spell effect. But PCs much more often get more Attacks, Spells, Bonus Action abilities, etc., than enemies, individually or (and this is important) in aggregate. They often also get specialized Reactions others than Opportunity Attack. This only starts to partially equalize when you get up to epic creatures that have legendary and lair actions, but even there, numbers tell.
• Everything you can do is part of your action economy: Actions, Bonus Actions, Reactions, and Moves. The more you can set yourself up (tactically, in a battle, or strategically, in your character design) to do something effective with all of those options in a turn, the more effective (and deadly) your character will be.

In short, the action economy is your range of actions in a round (see above), and by extension, maximizing your effectiveness by using as much of that economy as possible.

How do GMs cope?

GMs bitch a lot about this: the boss fight that’s got the arch-critter-demon you’ve had the players trembling about for months … ending with the boss going down in two rounds as the 15 attacks the party can generate per turn (action economy!) overwhelms the 4-5 the boss can.

What (just to offer notes) do GMs/module writers do in the face of this?

1. They add Minions! They’re not just color text — they help balance the “overwhelming numbers vs very powerful foe” equation by mitigating the former so that the latter can get some licks in.
2. As mentioned, epic-level legendary creatures — dragons, liches, beholders, etc. — can get legendary and/or lair powers, which basically add to their action economy (and hurt like the dickens). GMs often add these non-canonically to other bosses, too.
3. Do other things to add to a boss’s action economy. One suggestion that seems to have legs is making bosses, in short, multiple creatures (with different capabilities and HP pools and initiatives) presenting as a single creature.
4. Split the Party.   If the party can’t bring all of its power to bear — because it’s split up (by its own choosing or through an external force), or maybe because the attack vectors are limited (a narrow hallway, perhaps), it’s action economy is restrained.

The converse to all of these can be used (usually by the GM) to weaken a boss that seems too big to tackle.

Some good articles to check out:

• https://www.dungeonsolvers.com/2018/06/01/understanding-the-action-economy-in-dd-5e/
• https://tabletopjoab.com/action-economy-in-dd-5e-explained/
• https://www.cbr.com/dungeons-dragons-action-economy/

GM counter-tactics (as, ahem, thought experiments):

• https://www.reddit.com/r/DnDBehindTheScreen/comments/7bafqr/dd_5e_action_economy_identifying_the_problem/
• https://www.reddit.com/r/DnDBehindTheScreen/comments/7d97i5/dd_5e_action_economy_solutions/
• https://theangrygm.com/return-of-the-son-of-the-dd-boss-fight-now-in-5e/
• https://theangrygm.com/elemental-boogaloo/